J2EE Job Interview Companion

1 Learn Java/J2EE core concepts and design/coding issues

With

Java/J2EE Job Interview Companion

By

K.Arulkumaran

Technical Reviewers Craig Malone Lara D’Albreo Stuart Watson

Acknowledgements A. Sivayini R.Kumaraswamipillai

Cover Design K. Arulkumaran A.Sivayini

2 Java/J2EE Job Interview Companion Copy Right 2005 K.Arulkumaran The author has made every effort in the preparation of this book to ensure the accuracy of the information. However, information in this book is sold without warranty either express or implied. The author will not be held liable for any damages caused or alleged to be caused either directly or indirectly by this book.

3

Outline SECTION

DESCRIPTION What this book will do for you? Motivation for this book Key Areas index

SECTION 1

Interview questions and answers on: Java ƒ ƒ ƒ ƒ ƒ

SECTION 2

Language Fundamentals Swing Applet Performance and memory Leaks. Personal

Interview questions and answers on: Enterprise Java ƒ ƒ ƒ ƒ ƒ ƒ ƒ ƒ ƒ ƒ ƒ ƒ ƒ ƒ ƒ ƒ

SECTION 3

J2EE Servlet JSP JDBC JNDI RMI EJB JMS XML SQL, Database tuning and O/R mapping RUP & UML Struts Web and Application servers. Best practices and performance considerations. Testing and deployment. Personal

Putting it all together section. How would you go about…? 1.

How would you go about documenting your Java/J2EE application?

2.

How would you go about designing a Java/J2EE application?

3.

How would you go about identifying performance problems and/or memory leaks in your Java application?

4.

How would you go about minimising memory leaks in your Java/J2EE application?

5.

How would you go about improving performance of your Java/J2EE application?

6.

How would you go about identifying any potential thread-safety issues in your Java/J2EE application?

7.

How would you go about identifying any potential transactional issues in your Java/J2EE application?

8.

How would you go about applying the Object Oriented (OO) design concepts in your Java/J2EE

4 application? 9.

How would you go about applying the UML diagrams in your Java/J2EE project?

10. How would you go about describing the software development processes you are familiar with? 11. How would you go about applying the design patterns in your Java/J2EE application? 12. How would you go about determining the enterprise security requirements for your Java/J2EE application? 13. How would you go about describing the open source projects like JUnit (unit testing), Ant (build tool), CVS (version control system) and log4J (logging tool) which are integral part of most Java/J2EE projects? 14. How would you go about describing Web services? SECTION 4

SECTION 5

Emerging Technologies/Frameworks ƒ

Test Driven Development (TDD).

ƒ

Aspect Oriented Programming (AOP).

ƒ

Inversion of Control (IOC) (Also known as Dependency Injection).

ƒ

Annotations or attributes based programming (xdoclet etc).

ƒ

Spring framework.

ƒ

Hibernate framework.

ƒ

EJB 3.0.

ƒ

JavaServer Faces (JSF) framework.

Sample interview questions … ƒ

Java

ƒ

Web Components

ƒ

Enterprise

ƒ

Design

ƒ

General

GLOSSARY OF TERMS RESOURCES INDEX

5

Table of contents Outline_________________________________________________________________________________________ 3 Table of contents ________________________________________________________________________________ 5 What this book will do for you? ____________________________________________________________________ 7 Motivation for this book __________________________________________________________________________ 8 Key Areas Index ________________________________________________________________________________ 10 Java – Interview questions & answers _____________________________________________________________ 11 Java – Language Fundamentals ____________________________________________________________________________ 12 Java – Swing ____________________________________________________________________________________________ 44 Java – Applet____________________________________________________________________________________________ 48 Java – Performance and Memory leaks ______________________________________________________________________ 50 Java – Personal__________________________________________________________________________________________ 53 Java – Key Points ________________________________________________________________________________________ 56

Enterprise Java – Interview questions & answers ____________________________________________________ 59 Enterprise - J2EE ________________________________________________________________________________________ 60 Enterprise - Servlet _______________________________________________________________________________________ 69 Enterprise - JSP _________________________________________________________________________________________ 77 Enterprise - JDBC ________________________________________________________________________________________ 83 Enterprise – JNDI & LDAP _________________________________________________________________________________ 87 Enterprise - RMI _________________________________________________________________________________________ 90 Enterprise – EJB 2.x ______________________________________________________________________________________ 94 Enterprise - JMS ________________________________________________________________________________________ 110 Enterprise - XML ________________________________________________________________________________________ 114 Enterprise – SQL, Tuning and O/R mapping _________________________________________________________________ 119 Enterprise - RUP & UML __________________________________________________________________________________ 126 Enterprise - Struts_______________________________________________________________________________________ 133 Enterprise - Web and Application servers ___________________________________________________________________ 137 Enterprise - Best practices and performance considerations ___________________________________________________ 139 Enterprise – Logging, testing and deployment _______________________________________________________________ 141 Enterprise - Personal ____________________________________________________________________________________ 144 Enterprise – Software development process _________________________________________________________________ 144 Enterprise – Key Points __________________________________________________________________________________ 146

How would you go about…?_____________________________________________________________________ 151 Q 01:

How would you go about documenting your Java/J2EE application? ____________________________________ 152

Q 02:

How would you go about designing a Java/J2EE application? __________________________________________ 153

Q 03:

How would you go about identifying performance and/or memory issues in your Java/J2EE application? _____ 156

Q 04:

How would you go about minimising memory leaks in your Java/J2EE application? _______________________ 157

Q 05:

How would you go about improving performance in your Java/J2EE application? _________________________ 157

Q 06:

How would you go about identifying any potential thread-safety issues in your Java/J2EE application?_______ 158

Q 07:

How would you go about identifying any potential transactional issues in your Java/J2EE application? _______ 159

Q 08:

How would you go about applying the Object Oriented (OO) design concepts in your Java/J2EE application? _ 160

Q 09:

How would you go about applying the UML diagrams in your Java/J2EE project? _________________________ 162

6 Q 10:

How would you go about describing the software development processes you are familiar with? ____________163

Q 11:

How would you go about applying the design patterns in your Java/J2EE application? _____________________165

Q 12:

How would you go about determining the enterprise security requirements for yor Java/J2EE application? ____194

Q 13: How would you go about describing the open source projects like JUnit (unit testing), Ant (build tool), CVS (version control system) and log4J (logging tool) which are integral part of most Java/J2EE projects? ________________199 Q 14:

How would you go about describing Web services? __________________________________________________206

Emerging Technologies/Frameworks… ____________________________________________________________210 Q 01:

What is Test Driven Development (TDD)? ___________________________________________________________211

Q 02:

What is the point of Test Driven Development (TDD)? _________________________________________________211

Q 03:

What is aspect oriented programming? Explain AOP?_________________________________________________212

Q 04:

What are the differences between OOP and AOP? ____________________________________________________214

Q 05:

What are the benefits of AOP?_____________________________________________________________________214

Q 06:

What is attribute or annotation oriented programming?________________________________________________215

Q 07:

What are the pros and cons of annotations over XML based deployment descriptors?______________________215

Q 08:

What is XDoclet? ________________________________________________________________________________216

Q 09:

What is inversion of control (IOC) (also known as dependency injection)? ________________________________216

Q 10:

What are the different types of dependency injections?________________________________________________217

Q 11:

What are the benefits of IOC (aka Dependency Injection)? _____________________________________________217

Q 12:

What is the difference between a service locator pattern and an inversion of control pattern? _______________217

Q 13:

Why dependency injection is more elegant than a JNDI lookup to decouple client and the service? ___________218

Q 14:

Explain Object-to-Relational (O/R) mapping? ________________________________________________________218

Q 15:

Give an overview of hibernate framework? __________________________________________________________218

Q 16:

Explain some of the pitfalls of Hibernate and explain how to avoid them? ________________________________220

Q 17:

Give an overview of the Spring framework? _________________________________________________________221

Q 18:

How would EJB 3.0 simplify your Java development compared to EJB 1.x, 2.x? ___________________________222

Q 19:

Briefly explain key features of the JavaServer Faces (JSF) framework? __________________________________223

Q 20:

How would the JSF framework compare with the Struts framework?_____________________________________225

Sample interview questions… ____________________________________________________________________226 Java___________________________________________________________________________________________________227 Web components________________________________________________________________________________________227 Enterprise ______________________________________________________________________________________________227 Design_________________________________________________________________________________________________229 General ________________________________________________________________________________________________229

GLOSSARY OF TERMS__________________________________________________________________________230 RESOURCES __________________________________________________________________________________232 INDEX ________________________________________________________________________________________234

7

What this book will do for you? Have you got the time to read 10 or more books and articles to add value prior to the interview? This book has been written mainly from the perspective of Java/J2EE job seekers and interviewers. There are numerous books and articles on the market covering specific topics like Java, J2EE, EJB, Design Patterns, ANT, CVS, Multi-Threading, Servlets, JSP, emerging technologies like AOP (Aspect Oriented Programming), Test Driven Development (TDD), Inversion of Control (IoC) etc. But from an interview perspective it is not possible to brush up on all these books where each book usually has from 300 pages to 600 pages. The basic purpose of this book is to cover all the core concepts and design/coding issues which, all Java/J2EE developers, designers and architects should be conversant with to perform well in their current jobs and to launch a successful career by doing well at interviews. The interviewer can also use this book to make sure that they hire the right candidate depending on their requirements. This book contains a wide range of topics relating to Java/J2EE development in a concise manner supplemented with diagrams, tables, sample codes and examples. This book is also appropriately categorised to enable you to choose the area of interest to you. This book will assist all Java/J2EE practitioners to become better at what they do. Usually it takes years to understand all the core concepts and design/coding issues when you rely only on your work experience. The best way to fast track this is to read appropriate technical information and proactively apply these in your work environment. It worked for me and hopefully it will work for you as well. I was also at one stage undecided whether to name this book “Java/J2EE core concepts and solving design/coding issues” or “Java/J2EE Job Interview Companion”. The reason I chose “Java/J2EE Job Interview Companion” is because these core concepts and design/coding issues helped me to be successful in my interviews and also gave me thumbs up in code reviews.

8

Motivation for this book I started using Java in 1999 when I was working as a junior developer. During those two years as a permanent employee, I pro-actively spent many hours studying the core concepts behind Java/J2EE in addition to my hands on practical experience. Two years later I decided to start contracting. Since I started contracting in 2001, my career had a muchneeded boost in terms of contract rates, job satisfaction, responsibility etc. I moved from one contract to another with a view of expanding my skills and increasing my contract rates. In the last 5 years of contracting, I have worked for 5 different organisations both medium and large on 8 different projects. For each contract I held, on average I attended 6-8 interviews with different companies. In most cases multiple job offers were made and consequently I was in a position to negotiate my contract rates and also to choose the job I liked based on the type of project, type of organisation, technology used, etc. I have also sat for around 10 technical tests and a few preliminary phone interviews. The success in the interviews did not come easily. I spent hours prior to each set of interviews wading through various books and articles as a preparation. The motivation for this book was to collate all this information into a single book, which will save me time prior to my interviews but also can benefit others in their interviews. What is in this book has helped me to go from just a Java/J2EE job to a career in Java/J2EE in a short time. It has also given me the job security that ‘I can find a contract/permanent job opportunity even in the difficult job market’. I am not suggesting that every one should go contracting but by performing well at the interviews you can be in a position to pick the permanent role you like and also be able to negotiate your salary package. Those of you who are already in good jobs can impress your team leaders, solution designers and/or architects for a possible promotion by demonstrating your understanding of the key areas discussed in this book. You can discuss with your senior team members about performance issues, transactional issues, threading issues (concurrency issues) and memory issues. In most of my previous contracts I was in a position to impress my team leads and architects by pinpointing some of the critical performance, memory, transactional and threading issues with the code and subsequently fixing them. Trust me it is not hard to impress someone if you understand the key areas. For example: ƒ ƒ ƒ ƒ

Struts action classes are not thread-safe (Refer Q113 in Enterprise section). JSP variable declaration is not thread-safe (Refer Q34 in Enterprise section). Valuable resources like database connections should be closed properly to avoid any memory and performance issues (Refer Q45 in Enterprise section). Throwing an application exception will not rollback the transaction in EJB. (Refer Q77 in Enterprise section).

The other key areas, which are vital to any software development, are a good understanding of some of key design concepts, design patterns, and a modelling language like UML. These key areas are really worthy of a mention in your resume and interviews. For example: ƒ ƒ ƒ

Know how to use inheritance, polymorphism and encapsulation (Refer Q5, Q6, Q7, and Q8 in Java section.). Why use design patterns? (Refer Q5 in Enterprise section). Why is UML important? (Refer Q106 in Enterprise section).

If you happen to be in an interview with an organization facing serious issues with regards to their Java application relating to memory leaks, performance problems or a crashing JVM etc then you are likely to be asked questions on these topics. Refer Q 63 – Q 65 in Java section and Q123, Q125 in Enterprise section.

Another good reason why these key areas like transactional issues, design concepts, design patterns etc are vital are because solution designers, architects, team leads, and/or senior developers are usually responsible for conducting the technical interviews. These areas are their favourite topics because these are essential to any software development. Some interviewers request you to write a small program during interview or prior to getting to the interview stage. This is to ascertain that you can code using object oriented concepts and design patterns. So I have included a coding key area to illustrate what you need to look for while coding.

9 Apply OO concepts like inheritance, polymorphism and encapsulation: Refer Q08 in Java section. Program to interfaces not to implementations: Refer Q08, Q15 in Java section. Use of relevant design patterns: Refer Q11 in How would you go about… section. Use of Java collection API and exceptions correctly: Refer Q15, Q34, and Q35 in Java section. Stay away from hard coding values: Refer Q04 in Java section.

ƒ ƒ ƒ ƒ ƒ

L anguage F u n d a m e n t a ls

P e rfo rm a n c e Is s u e s

H o w m a n y b o o k s d o I h a v e to r e a d to u n d e r s t a n d a n d p u t t o g e t h e r a ll t h e s e k e y a re a s ? H o w m a n y y e a r s o f e x p e r ie n c e s h o u ld I h a v e t o u n d e r s t a n d a ll t h e s e k e y a re a s ?

S p e c if ic a t io n F u n d a m e n ta ls

W ill t h e s e k e y a r e a s h e lp m e p r o g r e s s in m y c a r e e r ?

S o ftw a re D e v e lo p m e n t P ro c e s s

W ill t h e s e k e y a r e a s h e lp m e c u t q u a lit y c o d e ?

D e s ig n P a tte rn s

E x c e p tio n H a n d lin g

D e s ig n C o n c e p ts

S E c u r ity T r a n s a c tio n a l Is s u e s

C

o n c u rre n c y

Is s u e s

CO

B est P r a c tic e s

S c a la b ilit y Iss u e s

M e m o ry Is su e s

d in g

LF

DC

CI

PI

SE

EH

SD

DP

SF

MI

SI

TI

BP

CO

This book aims to solve the above dilemma. My dad keeps telling me to find a permanent job (instead of contracting), which in his view provides better job security but I keep telling him that in my view in Information Technology the job security is achieved only by keeping your knowledge and skills sharp and up to date. The 8 contract positions I held over the last 5.5 years have given me broader experience in Java/J2EE and related technologies. It also kept me motivated since there was always something new to learn in each assignment, and not all companies will appreciate your skills and expertise until you decide to leave. Do the following statements sound familiar to you when you hand in your resignation or decide not to extend your contract after getting another job offer? “Can I tempt you to come back? What can I do to keep you here?” etc. You might even think why you waited so long. The best way to make an impression in any organisations is to understand and proactively apply and resolve the issues relating to the Key Areas discussed in the next section. But be a team player, be tactful and don’t be critical of everything, do not act in a superior way and have a sense of humour. “Technical skills must be complemented with interpersonal skills.”

Quick Read guide: It is recommended that you go through all the questions in all the sections but if you are pressed for time or would like to read it just before an interview then follow the steps shown below: 1. Read/Browse Popular Questions in Java and Enterprise Java sections. 2. Read/Browse Key Points in Java and Enterprise Java sections. 3. Read/Browse through “Emerging Technologies/Frameworks” section. 4. Read/Browse “How would you go about…” section excluding Q11 & Q13, which are discussed in detail.

10 Key Areas Index I have categorised the core concepts and issues into 14 key areas as listed below. These key areas are vital for any good software development. This index will enable you to refer to the questions based on key areas. Also note that each question has an icon next to it to indicate which key area or areas it belongs to. Additional reading is recommended for beginners in each of the key areas. Key Areas

icon

Question Numbers Java section

1

Q1-Q4, Q10-Q14, Q16Q20, Q22-Q27, Q30Q33, Q36-Q43, Q47-Q62 -

Enterprise section

Language Fundamentals

LF

Specification Fundamentals

SF

Design Concepts

DC

Q5-Q9, Q10, Q13, Q22, Q49

Design Patterns

DP

Q10, Q14, Q20, Q31, Q45, Q46, Q50, Q54, Q66

Transactional Issues

TI

Concurrency Issues

CI

Performance Issues

PI

Memory Issues

MI

Q22, Q29, Q32, Q33, Q36, Q45, Q64, Q65.

Scalability Issues Exception Handling Security

SI EH SE

Q19, Q20 Q34,Q35 Q61

Q20, Q21, Q120, Q122 Q76, Q77 Q12, Q13, Q23, Q35, Q46, Q51, Q58, Q81

Best Practices

BP

Software Development Process

SD

Q15, Q21, Q34, Q63, Q64 -

Q10, Q16, Q39, Q40, Q46, Q82, Q124, Q125 Q103-Q109, Q129, Q133, Q134, Q136

Coding1

CO

Q04, Q08, Q10, Q12, Q13, Q15, Q16, Q17, Q21, Q34, Q45, Q46

Q10, Q18, Q21, Q23, Q36, Q38, Q42, Q43, Q45, Q74, Q75, Q76, Q77, Q112, Q114, Q127, Q128

How would you go about…?

-

Emerging Technologies /Frameworks Q10, Q15, Q17, Q19

Q1-Q19, Q26-Q33, Q35Q38, Q41, Q42, Q44, Q46Q81, Q89-Q97, Q99, 102, Q110, Q112-Q115, Q118Q119, Q121, Q126, Q127, Q128 Q2, Q3, Q19, Q20, Q21, Q31, Q45, Q98, Q106, Q107, Q108, Q109, 101, Q111 Q5, Q5, Q22, Q24, Q25, Q83, Q84, Q85, Q86, Q87, Q88, Q110, Q111, Q116

Q14

-

Q43, Q71, Q72, Q73, Q74, Q75, Q77, Q78, Q79

Q7

Q13, Q15, Q29, Q36, Q40, Q53 Q13, Q15 -Q22, Q40, Q53, Q63.

Q16, Q34, Q113

Q6

Q10, Q16, Q43, Q45, Q46, Q72, Q83-Q88, Q97, Q98, Q100, Q102, Q123, Q125, Q128 Q45, Q93

Q3, Q5

Q02, Q08, Q09 Q11

Q3-Q9, Q11, Q13, Q14, Q16, Q18, Q20 Q12

Q3, Q4

Q12

Q1, Q10, Q13

Q1, Q2

Q11

Some interviewers request you to write a small program during interview or prior to getting to the interview stage. This is to ascertain that you can code using object oriented concepts and design patterns. I have included a coding key area to illustrate what you need to look for while coding. Unlike other key areas, the CO is not always shown against the question but shown above the actual section of relevance within a question.

Java

11

SECTION ONE

Java – Interview questions & answers

K E Y A R E A S

ƒ ƒ ƒ ƒ ƒ ƒ ƒ ƒ ƒ ƒ

Language Fundamentals Design Concepts DC Design Patterns DP Concurrency Issues CI Performance Issues PI Memory Issues MI Exception Handling EH Security SE Scalability Issues SI Coding1 CO

LF

Popular Questions: Q01, Q04, Q07, Q08, Q13, Q16, Q17, Q18, Q19, Q25, Q27, Q29, Q32, Q34, Q40, Q45, Q46, Q50, Q51, Q53, Q54, Q55, Q63,Q64, Q66, Q67

1

Unlike other key areas, the CO is not always shown against the question but shown above the actual subsection of relevance within a question.

Java

12

Java – Language Fundamentals

Q 01: Give a few reasons for using Java? LF DC A 01: Java is a fun language. Let’s look at some of the reasons: ƒ

Built-in support for multi-threading, socket communication, and memory management (automatic garbage collection).

ƒ

Object Oriented (OO).

ƒ

Better portability than other languages across operating systems.

ƒ

Supports Web based applications (Applet, Servlet, and JSP), distributed applications (sockets, RMI. EJB etc) and network protocols (HTTP, JRMP etc) with the help of extensive standardised APIs (Application Program Interfaces).

Q 02: What is the main difference between the Java platform and the other software platforms? LF A 02: Java platform is a software-only platform, which runs on top of other hardware-based platforms like UNIX, NT etc.

The Java platform has 2 components: ƒ

Java Virtual Machine (JVM) – ‘JVM’ is a software that can be ported onto various hardware platforms. Byte codes are the machine language of the JVM.

ƒ

Java Application Programming Interface (Java API) -

Q 03: What is the difference between C++ and Java? LF A 03: Both C++ and Java use similar syntax and are Object Oriented, but: ƒ

Java does not support pointers. Pointers are inherently tricky to use and troublesome.

ƒ

Java does not support multiple inheritances because it causes more problems than it solves. Instead Java supports multiple interface inheritance, which allows an object to inherit many method signatures from different interfaces with the condition that the inheriting object must implement those inherited methods. The multiple interface inheritance also allows an object to behave polymorphically on those methods. [Refer Q 8 and Q 10 in Java section.]

ƒ

Java does not support destructors but rather adds a finalize() method. Finalize methods are invoked by the garbage collector prior to reclaiming the memory occupied by the object, which has the finalize() method. This means you do not know when the objects are going to be finalized. Avoid using finalize() method to release non-memory resources like file handles, sockets, database connections etc because Java has only a finite number of these resources and you do not know when the garbage collection is going to kick in to release these resources through the finalize() method.

ƒ

Java does not include structures or unions because the traditional data structures are implemented as an object oriented framework (Java collection framework – Refer Q14, Q15 in Java section).

Java

13

ƒ

All the code in Java program is encapsulated within classes therefore Java does not have global variables or functions.

ƒ

C++ requires explicit memory management, while Java includes automatic garbage collection. [Refer Q32 in Java section].

Q 04: Explain Java class loaders? Explain dynamic class loading? LF A 04: Class loaders are hierarchical. Classes are introduced into the JVM as they are referenced by name in a class that is already running in the JVM. So how is the very first class loaded? The very first class is specially loaded with the help of static main() method declared in your class. All the subsequently loaded classes are loaded by the classes, which are already loaded and running. A class loader creates a namespace. All JVMs include at least one class loader that is embedded within the JVM called the primordial (or bootstrap) class loader. Now let’s look at non-primordial class loaders. The JVM has hooks in it to allow user defined class loaders to be used in place of primordial class loader. Let us look at the class loaders created by the JVM. CLASS LOADER Bootstrap (primordial)

reloadable? No

Explanation Loads JDK internal classes, java.* packages. (as defined in the sun.boot.class.path system property, typically loads rt.jar and i18n.jar)

Extensions

No

Loads jar files from JDK extensions directory (as defined in the java.ext.dirs system property – usually lib/ext directory of the JRE)

System

No

Loads classes from system classpath (as defined by the java.class.path property, which is set by the CLASSPATH environment variable or –classpath or –cp command line options)

JVM class loaders Bootstrap (primordial) (rt.jar, i18.jar)

The classes loaded by system class loader have visibility into classes loaded by its parents (ie Extensions and Bootstrap class loaders).

Extensions (lib/ext)

If there were any sibling class loaders they cannot see classes loaded by each other. T hey can only see the classes loaded by their parent class loader. For example Sibling1 class loader cannot see classes loaded by Sibling2 class loader

System (-classpath) Sibling1 classloader

Classes loaded by Bootstrap class loader have no visibility into classes loaded by its descendants (ie Extensions and Systems class loaders).

Sibling2 classloader

Both Sibling1 and Sibling2 class loaders have visibilty into classes loaded by their parent class loaders (eg: System, Extensions, and Bootstrap)

Class loaders are hierarchical and use a delegation model when loading a class. Class loaders request their parent to load the class first before attempting to load it themselves. When a class loader loads a class, the child class loaders in the hierarchy will never reload the class again. Hence uniqueness is maintained. Classes loaded by a child class loader have visibility into classes loaded by its parents up the hierarchy but the reverse is not true as explained in the above diagram. Important: Two objects loaded by different class loaders are never equal even if they carry the same values, which mean a class is uniquely identified in the context of the associated class loader. This applies to singletons too, where each class loader will have its own singleton. [Refer Q45 in Java section for singleton design pattern]

Explain static vs. dynamic class loading? Static class loading Dynamic class loading Classes are statically loaded with Java’s “new” operator.

Dynamic loading is a technique for programmatically invoking the functions of a class loader at run time. Let us look at how to load classes dynamically.

class MyClass { public static void main(String args[]) { Car c = new Car(); } }

Class.forName (String className); //static method which returns a Class The above static method returns the class object associated with the class name. The string className can be supplied dynamically at run time. Unlike the static loading, the dynamic loading will decide whether to load the class Car or the class Jeep at runtime based on a properties file and/or other runtime

Java

14

conditions. Once the class is dynamically loaded the following method returns an instance of the loaded class. It’s just like creating a class object with no arguments. class.newInstance (); //A non-static method, which creates an instance of a class (i.e. creates an object). Jeep myJeep = null ; //myClassName should be read from a properties file or Constants interface. //stay away from hard coding values in your program. CO String myClassName = "au.com.Jeep" ; Class vehicleClass = Class.forName(myClassName) ; myJeep = (Jeep) vehicleClass.newInstance(); myJeep.setFuelCapacity(50); A NoClassDefFoundException is thrown if a class is referenced with Java’s “new” operator (i.e. static loading) but the runtime system cannot find the referenced class.

A ClassNotFoundException is thrown when an application tries to load in a class through its string name using the following methods but no definition for the class with the specified name could be found: ƒ ƒ ƒ

The forName(..) method in class - Class. The findSystemClass(..) method in class - ClassLoader. The loadClass(..) method in class - ClassLoader.

What are “static initializers” or “static blocks with no function names”? When a class is loaded, all blocks that are declared static and don’t have function name (i.e. static initializers) are executed even before the constructors are executed. As the name suggests they are typically used to initialize static fields. CO public class StaticInitilaizer { public static final int A = 5; public static final int B; //Static initializer block, which is executed only once when the class is loaded. static { if(A == 5) B = 10; else B = 5; } public StaticInitilaizer(){} // constructor is called only after static initializer block } The following code gives an Output of A=5, B=10. public class Test { System.out.println("A =" + StaticInitilaizer.A + ", B =" + StaticInitilaizer.B); }

Q 05: What are the advantages of Object Oriented Programming Languages (OOPL)? DC A 05: The Object Oriented Programming Languages directly represent the real life objects like Car, Jeep, Account, Customer etc. The features of the OO programming languages like polymorphism, inheritance and encapsulation make it powerful. [Tip: remember pie which, stands for Polymorphism, Inheritance and Encapsulation are the 3 pillars of OOPL]

Q 06: How does the Object Oriented approach improve software development? DC A 06: The key benefits are: ƒ

Re-use of previous work: using implementation inheritance and object composition.

ƒ

Real mapping to the problem domain: Objects map to real world and represent vehicles, customers, products etc: with encapsulation.

ƒ

Modular Architecture: Objects, systems, frameworks etc are the building blocks of larger systems.

Java

15

The increased quality and reduced development time are the by-products of the key benefits discussed above. If 90% of the new application consists of proven existing components then only the remaining 10% of the code have to be tested from scratch.

Q 07: How do you express an ‘is a’ relationship and a ‘has a’ relationship or explain inheritance and composition? What is the difference between composition and aggregation? DC

A 07: The ‘is a’ relationship is expressed with inheritance and ‘has a’ relationship is expressed with composition. Both inheritance and composition allow you to place sub-objects inside your new class. Two of the main techniques for code reuse are class inheritance and object composition.

Inheritance [ is a ] Vs Composition [ has a ] Building

House

is a

is a [House is a Building]

has a

class Building{ ....... }

Bathroom

class House extends Building{ ......... }

has a [House has a Bathroom] class House { Bathroom room = new Bathroom() ; .... public void getTotMirrors(){ room.getNoMirrors(); .... } }

Inheritance is uni-directional. For example House is a Building. But Building is not a House. Inheritance uses extends key word. Composition: is used when House has a Bathroom. It is incorrect to say House is a Bathroom. Composition simply means using instance variables that refer to other objects. The class House will have an instance variable, which refers to a Bathroom object. Which one to use? The guide is that inheritance should be only used when subclass ‘is a’ superclass. ƒ

Don’t use inheritance just to get code reuse. If there is no ‘is a’ relationship then use composition for code reuse. Overuse of implementation inheritance (uses the “extends” key word) can break all the subclasses, if the superclass is modified.

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Do not use inheritance just to get polymorphism. If there is no ‘is a’ relationship and all you want is polymorphism then use interface inheritance with composition, which gives you code reuse (Refer Q8 in Java section for interface inheritance).

What is the difference between aggregation and composition? Aggregation Composition Aggregation is an association in which one class belongs to a collection. This is a part of a whole relationship where a part can exist without a whole. For example a line item is a whole and product is a part. If a line item is deleted then corresponding product need not be deleted. So aggregation has a weaker relationship.

Composition is an association in which one class belongs to a collection. This is a part of a whole relationship where a part cannot exist without a whole. If a whole is deleted then all parts are deleted. For example An order is a whole and line items are parts. If an order deleted then all corresponding line items for that order should be deleted. So composition has a stronger relationship.

Q 08: What do you mean by polymorphism, inheritance, encapsulation, and dynamic binding? DC A 08: Polymorphism – means the ability of a single variable of a given type to be used to reference objects of different types, and automatically call the method that is specific to the type of object the variable references. In a nutshell, polymorphism is a bottom-up method call. The benefit of polymorphism is that it is very easy to add new classes of derived objects without breaking the calling code (i.e. getTotArea() in the sample code shown below) that uses the polymorphic classes or interfaces. When you send a message to an object even though you don’t know what specific type it is, and the right thing happens, that’s called polymorphism. The process used by objectoriented programming languages to implement polymorphism is called dynamic binding. Let us look at some sample code to demonstrate polymorphism: CO

Java

16 Sample code: //client or calling code double dim = 5.0; //ie 5 meters radius or width List listShapes = new ArrayList(20); Shape s = new Circle(); listShapes.add(s); //add circle s = new Square(); listShapes.add(s); //add square getTotArea (listShapes,dim); //returns 78.5+25.0=103.5

//Later on, if you decide to add a half circle then define //a HalfCircle class, which extends Circle and then provide an //area(). method but your called method getTotArea(...) remains //same. s = new HalfCircle(); listShapes.add(s); //add HalfCircle getTotArea (listShapes,dim); //returns 78.5+25.0+39.25=142.75 /** called method: method which adds up areas of various ** shapes supplied to it. **/ public double getTotArea(List listShapes, double dim){ Iterator it = listShapes.iterator(); double totalArea = 0.0; //loop through different shapes while(it.hasNext()) { Shape s = (Shape) it.next(); totalArea += s.area(dim); //polymorphic method call } return totalArea ; }

For example: given a base class/interface Shape, polymorphism allows the programmer to define different area(double dim1) methods for any number of derived classes such as Circle, Square etc. No matter what shape an object is, applying the area method to it will return the right results. Later on HalfCicle can be added without breaking your called code i.e. method getTotalArea(...) Depending on what the shape is, appropriate area(double dim) method gets called and calculated. Circle Æ area is 78.5sqm Square Æ area is 25sqm HalfCircle Æ area is 39.25 sqm

Shape +area() : double

Circle

Square

+area() : double

+area() : double

HalfCircle +area() : double

«interface» Shape +area() : double

Circle

Square

+area() : double

+area() : double

HalfCircle +area() : double

Inheritance – is the inclusion of behaviour (i.e. methods) and state (i.e. variables) of a base class in a derived class so that they are accessible in that derived class. The key benefit of Inheritance is that it provides the formal mechanism for code reuse. Any shared piece of business logic can be moved from the derived class into the base class as part of refactoring process to improve maintainability of your code by avoiding code duplication. The existing class is called the superclass and the derived class is called the subclass. Inheritance can also be defined as the process whereby one object acquires characteristics from one or more other objects the same way children acquire characteristics from their parents. There are two types of inheritances: 1. Implementation inheritance (aka class inheritance): You can extend an applications’ functionality by reusing functionality in the parent class by inheriting all or some of the operations already implemented. In Java, you can only inherit from one superclass. Implementation inheritance promotes reusability but improper use of class inheritance can cause programming nightmares by breaking encapsulation and making future changes a problem. With implementation inheritance, the subclass becomes tightly coupled with the superclass. This will make the design fragile because if you want to change the superclass, you must know all the details of the subclasses to avoid breaking them. So when using implementation inheritance, make sure that the subclasses depend only on the behaviour of the superclass, not on the actual implementation. For example in the above diagram the subclasses should only be concerned about the behaviour known as area() but not how it is implemented. 2. Interface inheritance (aka type inheritance): This is also known as subtyping. Interfaces provide a mechanism for specifying a relationship between otherwise unrelated classes, typically by specifying a set of common methods each implementing class must contain. Interface inheritance promotes the design concept of program to interfaces not to implementations. This also reduces the coupling or implementation dependencies between systems. In Java, you can implement any number of interfaces. This is more flexible than implementation inheritance because it won’t lock you into specific implementations which make subclasses difficult to maintain. So care should be taken not to break the implementing classes by modifying the interfaces. Which one to use? Prefer interface inheritance to implementation inheritance because it promotes the design concept of coding to an interface and reduces coupling. Interface inheritance can achieve code reuse with the help of object composition. If you look at Gang of Four (GoF) design patterns, you can see that it favours interface inheritance to implementation inheritance. CO

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Implementation inheritance

Interface inheritance

Let’s assume that savings account and term deposit account have a similar behaviour in terms of depositing and withdrawing money, so we will get the super class to implement this behaviour and get the subclasses to reuse this behaviour. But saving account and term deposit account have specific behaviour in calculating the interest.

Let’s look at an interface inheritance code sample, which makes use of composition for reusability. In the following example the methods deposit(…) and withdraw(…) share the same piece of code in AccountHelper class. The method calculateInterest(…) has its specific implementation in its own class.

public abstract class Account { public void deposit(double amount) { //deposit logic } public void withdraw(double amount) { //withdraw logic } public abstract double calculateInterest(double amount);

public interface Account { public abstract void deposit(double amount); public abstract void withdraw(double amount); public abstract int getAccountType(); } public interface SavingsAccount extends Account{ public abstract double calculateInterest(double amount); } public interface TermDepositAccount extends Account{ public abstract double calculateInterest(double amount); }

} public class SavingsAccount extends Account { public double calculateInterest(double amount) { //calculate interest for SavingsAccount } }

The classes SavingsAccountImpl, TermDepositAccountImpl should implement the methods declared in its interfaces. The class AccountHelper implements the methods deposit(…) and withdraw(…) public class SavingsAccountImpl implements SavingsAccount{ private int accountType = 1;

public class TermDepositAccount extends Account {

//helper class which promotes code reuse through composition AccountHelper helper = new AccountHelper();

public double calculateInterest(double amount) { //calculate interest for TermDeposit }

public void deposit(double amount) { helper.deposit(amount, getAccountType()); } public void withdraw(double amount) { helper.withdraw(amount, getAccountType()); } public double calculateInterest(double amount) { //calculate interest for SavingsAccount } public int getAccountType(){ return accountType; }

} The calling code can be defined as follows for illustration purpose only: public class Test { public static void main(String[] args) { Account acc1 = new SavingsAccount(); acc1.deposit(5.0); acc1.withdraw(2.0); Account acc2 = new TermDepositAccount(); acc2.deposit(10.0); acc2.withdraw(3.0);

} public class TermDepositAccountImpl implements TermDepositAccount { private int accountType = 2;

acc1.calculateInterest(500.00); acc2.calculateInterest(500.00);

//helper class which promotes code reuse through composition AccountHelper helper = new AccountHelper();

} }

public void deposit(double amount) { helper.deposit(amount, getAccountType()); } public void withdraw(double amount) { helper.withdraw(amount, getAccountType()); } public double calculateInterest(double amount) { //calculate interest for TermDeposit } public int getAccountType() { return accountType; } } The calling code can be defined as follows for illustration purpose only: public class Test { public static void main(String[] args) {

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Account acc1 = new SavingsAccountImpl(); acc1.deposit(5.0); Account acc2 = new TermDepositAccountImpl(); acc2.deposit(10.0); if (acc1.getAccountType() == 1) { ((SavingsAccount) acc1).calculateInterest(500.00); } if (acc2.getAccountType() == 2) { ((TermDepositAccount) acc2).calculateInterest(500.00); } } }

Encapsulation – refers to keeping all the related members (variables and methods) together in an object. Specifying members as private can hide the variables and methods. Objects should hide their inner workings from the outside view. Good encapsulation improves code modularity by preventing objects interacting with each other in an unexpected way, which in turn makes future development and refactoring efforts easy. Being able to encapsulate members of a class is important for security and integrity. We can protect variables from unacceptable values. The sample code below describes how encapsulation can be used to protect the MyMarks object from having negative values. Any modification to member variable “vmarks” can only be carried out through the setter method setMarks(int mark). This prevents the object “MyMarks” from having any negative values by throwing an exception. CO

Sample code Class MyMarks { private int vmarks = 0; private String name; public void setMarks(int mark) throws MarkException { if(mark > 0) this.vmarks = mark; else { throw new MarkException("No negative Values"); } } public int getMarks(){ return vmarks; } //getters and setters for attribute name goes here.

k) ar m t in s( k ar tM e s

in tg et M ar ks ()

private int vmarks; private String name;

se tN am e

(S tri ng

na m e)

g rin St

Member variables are encapsulated, so that they can only be accessed via encapsulating methods.

) e( m a tN ge

}

Q 09: What is design by contract? Explain the assertion construct? DC A 09: Design by contract specifies the obligations of a calling-method and called-method to each other. Design by contract is a valuable technique, which should be used to build well-defined interfaces. The strength of this programming methodology is that it gets the programmer to think clearly about what a function does, what pre and post conditions it must adhere to and also it provides documentation for the caller. Java uses the assert statement to implement pre- and post-conditions. Java’s exceptions handling also support design by contract especially checked exceptions (Refer Q34 in Java section for checked exceptions). In design by contract in addition to specifying programming code to carrying out intended operations of a method the programmer also specifies: 1. Preconditions – This is the part of the contract the calling-method must agree to. Preconditions specify the conditions that must be true before a called method can execute. Preconditions involve the system state and the arguments passed into the method at the time of its invocation. If a precondition fails then there is a bug in the calling-method or calling software component.

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On public methods

On non-public methods

Preconditions on public methods are enforced by explicit checks that throw particular, specified exceptions. You should not use assertion to check the parameters of the public methods but can use for the non-public methods. Assert is inappropriate because the method guarantees that it will always enforce the argument checks. It must check its arguments whether or not assertions are enabled. Further, assert construct does not throw an exception of a specified type. It can throw only an AssertionError.

You can use assertion to check the parameters of the non-public methods.

public void setRate(int rate) { if(rate MAX_RATE){ throw new IllegalArgumentException(“Invalid rate Æ ” + rate); } setCalculatedRate(rate); }

private void setCalculatedRate(int rate) { assert (rate > 0 && rate < MAX_RATE) : rate; //calculate the rate and set it. }

Assertions can be disabled, so programs must not assume that assert construct will be always executed: //Wrong: if assertion is disabled, CarpenterJob never //Get removed assert jobsAd.remove(PilotJob); //Correct: boolean pilotJobRemoved = jobsAd.remove(PilotJob); assert pilotJobRemoved;

2. Postconditions – This is the part of the contract the called-method agrees to. What must be true after a method completes successfully. Postconditions can be used with assertions in both public and non-public methods. The postconditions involve the old system state, the new system state, the method arguments and the method’s return value. If a postcondition fails then there is a bug in the called-method or called software component. public double calcRate(int rate) { if(rate MAX_RATE){ throw new IllegalArgumentException(“Invalid rate !!! ”); } //logic to calculate the rate and set it goes here assert this.evaluate(result) < 0 : this; //this Æ message sent to AssertionError on failure return result;

}

3. Class invariants - what must be true about each instance of a class? A class invariant as an internal invariant that can specify the relationships among multiple attributes, and should be true before and after any method completes. If an invariant fails then there could be a bug in either calling-method or called-method. There is no particular mechanism for checking invariants but it is convenient to combine all the expressions required for checking invariants into a single internal method that can be called by assertions. For example if you have a class, which deals with negative integers then you define the isNegative() convenient internal method: class NegativeInteger { Integer value = new Integer (-1); //invariant //constructor public NegativeInteger(Integer int) { //constructor logic goes here assert isNegative(); } //rest of the public and non-public methods goes here. public methods should call assert isNegative(); prior to its return //convenient internal method for checking invariants. Returns true if the integer value is negative private boolean isNegative(){ return value.intValue() < 0 ; } }

The isNegative() method should be true before and after any method completes, each public method and constructor should contain the following assert statement immediately prior to its return. assert isNegative();

Explain the assertion construct? The assertion statements have two forms as shown below: assert Expression1;

Java

20 assert Expression1 : Expression2;

Where: ƒ ƒ

Expression1 Æ is a boolean expression. If the Expression1 evaluates to false, it throws an AssertionError without any detailed message. Expression2 Æ if the Expression1 evaluates to false throws an AssertionError with using the value of the Expression2 as the errors’ detailed message.

Note: If you are using assertions (available from JDK1.4 onwards), you should supply the JVM argument to enable it by package name or class name. Java -ea[:packagename...|:classname] or Java -enableassertions[:packagename...|:classname] Java –ea:Account

Q 10: What is the difference between an abstract class and an interface and when should you use them? LF DP DC A 10: In design, you want the base class to present only an interface for its derived classes. This means, you don’t want anyone to actually instantiate an object of the base class. You only want to upcast to it (implicit upcasting, which gives you polymorphic behaviour), so that its interface can be used. This is accomplished by making that class abstract using the abstract keyword. If anyone tries to make an object of an abstract class, the compiler prevents it. The interface keyword takes this concept of an abstract class a step further by preventing any method or function implementation at all. You can only declare a method or function but not provide the implementation. The class, which is implementing the interface, should provide the actual implementation. The interface is a very useful and commonly used aspect in OO design, as it provides the separation of interface and implementation and enables you to: ƒ ƒ ƒ ƒ

Capture similarities among unrelated classes without artificially forcing a class relationship. Declare methods that one or more classes are expected to implement. Reveal an object's programming interface without revealing its actual implementation. Model multiple interface inheritance in Java, which provides some of the benefits of full on multiple inheritances, a feature that some object-oriented languages support that allow a class to have more than one superclass. Diam ond problem & use of interface ShapeIF

Shape

C ircle

Square

C ircleO nSquare

Square

C ircle

C ircleIF

SquareIF

C ircleO nSquare M ultiple interface inheritance in JA VA N o m ultiple inheritance in JA VA

Abstract class

Interface

Have executable methods and abstract methods.

Have no implementation code. All methods are abstract.

Can only subclass one abstract class.

A class can implement any number of interfaces.

Can have instance variables, constructors and any visibility: public, private, protected, none (aka package).

Cannot have instance variables, constructors and can have only public and none (aka package) visibility.

When to use an abstract class?: In case where you want to use implementation inheritance then it is usually provided by an abstract base class. Abstract classes are excellent candidates inside of application frameworks. Abstract classes let you define some default behaviour and force subclasses to provide any specific behaviour. Care should be taken not to overuse implementation inheritance as discussed in Q8 in Java section.

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When to use an interface?: For polymorphic interface inheritance, where the client wants to only deal with a type and does not care about the actual implementation use interfaces. If you need to change your design frequently, you should prefer using interface to abstract. CO Coding to an interface reduces coupling and interface inheritance can achieve code reuse with the help of object composition. Another justification for using interfaces is that they solve the ‘diamond problem’ of traditional multiple inheritance as shown in the figure. Java does not support multiple inheritances. Java only supports multiple interface inheritance. Interface will solve all the ambiguities caused by this ‘diamond problem’. Design pattern: Strategy design pattern lets you swap new algorithms and processes into your program without altering the objects that use them. Strategy design pattern: Refer Q11 in How would you go about… section.

Q 11: Why there are some interfaces with no defined methods (i.e. marker interfaces) in Java? LF A 11: The interfaces with no defined methods act like markers. They just tell the compiler that the objects of the classes implementing the interfaces with no defined methods need to be treated differently. Example Serializable (Refer Q19 in Java section), Cloneable etc

Q 12: When is a method said to be overloaded and when is a method said to be overridden? LF CO A 12: Method Overloading

Method Overriding

Overloading deals with multiple methods in the same class with the same name but different method signatures.

Overriding deals with two methods, one in the parent class and the other one in the child class and has the same name and signatures.

class MyClass { public void getInvestAmount(int rate) {…}

class BaseClass{ public void getInvestAmount(int rate) {…} }

public void getInvestAmount(int rate, long principal) { … } } Both the above methods have the same method names but different method signatures, which mean the methods are overloaded.

Overloading lets you define the same operation in different ways for different data.

class MyClass extends BaseClass { public void getInvestAmount(int rate) { …} } Both the above methods have the same method names and the signatures but the method in the subclass MyClass overrides the method in the superclass BaseClass. Overriding lets you define the same operation in different ways for different object types.

Q 13: What is the main difference between an ArrayList and a Vector? What is the main difference between Hashmap and Hashtable? LF DC PI CI

A 13: Vector / Hashtable Original classes before the introduction of Collections API. Vector & Hashtable are synchronized. Any method that touches their contents is thread-safe.

ArrayList / Hashmap So if you don’t need a thread safe collection, use the ArrayList or Hashmap. Why pay the price of synchronization unnecessarily at the expense of performance degradation.

So which is better? As a general rule, prefer ArrayList/Hashmap to Vector/Hashtable. If your application is a multithreaded application and at least one of the threads either adds or deletes an entry into the collection then use new Java collection API‘s external synchronization facility as shown below to temporarily synchronize your collections as needed: CO Map myMap = Collections.synchronizedMap (myMap); List myList = Collections.synchronizedList (myList);

Java arrays are even faster than using an ArrayList/Vector and perhaps therefore may be preferable. ArrayList/Vector internally uses an array with some convenient methods like add(..), remove(…) etc.

Q 14: Explain the Java Collection framework? LF DP

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A 14: The key interfaces used by the collection framework are List, Set and Map. The List and Set extends the Collection interface. Should not confuse the Collection interface with the Collections class which is a utility class. A Set is a collection with unique elements and prevents duplication within the collection. HashSet and TreeSet are implementations of a Set interface. A List is a collection with an ordered sequence of elements and may contain duplicates. ArrayList, LinkedList and Vector are implementations of a List interface. The Collection API also supports maps, but within a hierarchy distinct from the Collection interface. A Map is an object that maps keys to values, where the list of keys is itself a collection object. A map can contain duplicate values, but the keys in a map must be distinct. HashMap, TreeMap and Hashtable are implementations of a Map interface. How to implement collection ordering? SortedSet and SortedMap interfaces maintain sorted order. The classes, which implement the Comparable interface, impose natural order. For classes that don’t implement comparable interface, or when one needs even more control over ordering based on multiple attributes, a Comparator interface should be used. Design pattern: What is an Iterator? An Iterator is a use once object to access the objects stored in a collection. Iterator design pattern (aka Cursor) is used, which is a behavioural design pattern that provides a way to access elements of a collection sequentially without exposing its internal representation. JAVA collectio n fram ework < in terface> C o llectio n

java.u til.C o llectio n s

A b stractC o llectio n

ex tends < in terface> L ist

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A rrays A b stractL ist

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< in terface> S et

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A b stractS et A b stract S eq u en tial L ist

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im plements

Id en tity H ash M ap

T reeS et

T reeM ap

A rrayL ist V ecto r

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S tack

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W eak H ash M ap

H ash M ap L in ked H ash M ap

(Diagram sourced from: http://www.wilsonmar.com/1arrays.htm) What are the benefits of the Java collection framework? Collection framework provides flexibility, performance, and robustness. ƒ ƒ ƒ ƒ ƒ ƒ

Polymorphic algorithms – sorting, shuffling, reversing, binary search etc. Set algebra - such as finding subsets, intersections, and unions between objects. Performance - collections have much better performance compared to the older Vector and Hashtable classes with the elimination of synchronization overheads. Thread-safety - when synchronization is required, wrapper implementations are provided for temporarily synchronizing existing collection objects. Immutability - when immutability is required wrapper implementations are provided for making a collection immutable. Extensibility - interfaces and abstract classes provide an excellent starting point for adding functionality and features to create specialized object collections.

Q 15: What are some of the best practices relating to Java collection? BP PI CI A 15: ƒ

Use ArrayLists, HashMap etc as opposed to Vector, Hashtable etc, where possible to avoid any synchronization overhead. Even better is to use just arrays where possible. If multiple threads concurrently access a collection and at least one of the threads either adds or deletes an entry into the collection, then the collection must be externally synchronized. This is achieved by: Map myMap = Collections.synchronizedMap (myMap);

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List myList = Collections.synchronizedList (myList);

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Set the initial capacity of a collection appropriately (e.g. ArrayList, HashMap etc). This is because collection classes like ArrayList, HashMap etc must grow periodically to accommodate new elements. But if you have a very large array, and you know the size in advance then you can speed things up by setting the initial size appropriately. For example: HashMaps/Hashtables need to be created with sufficiently large capacity to minimise rehashing (which happens every time the table grows). HashMap has two parameters initial capacity and load factor that affect its performance and space requirements. Higher load factor values (default load factor of 0.75 provides a good trade off between performance and space) will reduce the space cost but will increase the lookup cost of myMap.get(…) and myMap.put(…) methods. When the number of entries in the HashMap exceeds the current capacity * loadfactor then the capacity of the HasMap is roughly doubled by calling the rehash function. It is also very important not to set the initial capacity too high or load factor too low if iteration performance or reduction in space is important.

ƒ

Program in terms of interface not implementation: For example you might decide a LinkedList is the best choice for some application, but then later decide ArrayList might be a better choice for performance reason. CO Use: List list = new ArrayList(100); //program in terms of interface & set the initial capacity. Instead of: ArrayList list = new ArrayList();

ƒ

Avoid storing unrelated or different types of objects into same collection: This is analogous to storing items in pigeonholes without any labelling. To store items use value objects or data objects (as oppose to storing every attribute in an ArrayList or HashMap). Provide wrapper classes around your collection API classes like ArrayList, Hashmap etc as shown in better approach column. Also where applicable consider using composite design pattern, where an object may represent a single object or a collection of objects. Refer Q52 in Java section for UML diagram of a composite design pattern. CO

Avoid where possible

Better approach

The code below is hard to maintain and understand by others. Also gets more complicated as the requirements grow in the future because we are throwing different types of objects like Integer, String etc into a list just based on the indices and it is easy to make mistakes while casting the objects back during retrieval.

When storing items into a collection define value objects as shown below: (VO is an acronym for Value Object). public class LineItemVO { private int itemId; private String productName;

List myOrder = new ArrayList()

public int getLineItemId(){return accountId ;} public int getAccountName(){return accountName;}

ResultSet rs = …

public void setLineItemId(int accountId ){ this.accountId = accountId } //implement other getter & setter methods

While (rs.hasNext()) { List lineItem = new ArrayList(); lineItem.add (new Integer(rs.getInt(“itemId”))); lineItem.add (rs.getString(“description”)); …. myOrder.add( lineItem); } return myOrder;

} Now let’s define our base wrapper class, which represents an order: public abstract class Order { int orderId; List lineItems = null; public abstract int countLineItems(); public abstract boolean add(LineItemVO itemToAdd); public abstract boolean remove(LineItemVO itemToAdd); public abstract Iterator getIterator(); public int getOrderId(){return this.orderId; }

Example 2:

List myOrder = new ArrayList(10); //create an order OrderVO header = new OrderVO(); header.setOrderId(1001); … //add all the line items LineItemVO line1 = new LineItemVO(); line1.setLineItemId(1); LineItemVO line2 = new LineItemVO(); Line2.setLineItemId(2);

} Now a specific implementation of our wrapper class: public class OverseasOrder extends Order { public OverseasOrder(int inOrderId) { this.lineItems = new ArrayList(10); this.orderId = inOrderId; }

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List lineItems = new ArrayList(); lineItems.add(line1); lineItems.add(line2);

public int countLineItems() { //logic to count } public boolean add(LineItemVO itemToAdd){ …//additional logic or checks return lineItems.add(itemToAdd); }

//to store objects myOrder.add(order);// index 0 is an OrderVO object myOrder.add(lineItems);//index 1 is a List of line items

public boolean remove(LineItemVO itemToAdd){ return lineItems.remove(itemToAdd); }

//to retrieve objects myOrder.get(0); myOrder.get(1);

public ListIterator getIterator(){ return lineItems.Iterator();} Above approaches are bad because disparate objects are stored in the lineItem collection in example-1 and example-2 relies on indices to store disparate objects. The indices based approach and storing disparate objects are hard to maintain and understand because indices are hard coded and get scattered across the code. If an index position changes for some reason, then you will have to change every occurrence, otherwise it breaks your application.

}

The above coding approaches are analogous to storing disparate items in a storage system without proper labelling and just relying on its grid position.

LineItemVO item2 = new LineItemVO(); Item1.setItemId(2); Item1.setProductName(“Outdoor chair”);

Now to use: Order myOrder = new OverseasOrder(1234) ; LineItemVO item1 = new LineItemVO(); Item1.setItemId(1); Item1.setProductName(“BBQ”);

//to add line items to order myOrder.add(item1); myOrder.add(item2); …

Q 16: When providing a user defined key class for storing objects in the Hashmaps or Hashtables, what methods do you have to provide or override (i.e. method overriding)? LF PI CO

A 16: You should override the equals() and hashCode() methods from the Object class. The default implementation of the equals() and hashcode(), which are inherited from the java.lang.Object uses an object instance’s memory location (e.g. MyObject@6c60f2ea). This can cause problems when two instances of the car objects have the same colour but the inherited equals() will return false because it uses the memory location, which is different for the two instances. Also the toString() method can be overridden to provide a proper string representation of your object. Points to consider: • • • •

If a class overrides equals(), it must override hashCode(). If 2 objects are equal, then their hashCode values must be equal as well. If a field is not used in equals(), then it must not be used in hashCode(). If it is accessed often, hashCode() is a candidate for caching to enhance performance.

Note: Java 1.5 introduces enumerated constants, which improves readability and maintainability of your code. Java programming language enums are more powerful than their counterparts in other languages. E.g. A class like Weather can be built on top of simple enum type Season and the class Weather can be made immutable, and only one instance of each Weather can be created, so that your Weather class does not have to override equals() and hashCode() methods. public class Weather { public enum Season {WINTER, SPRING, SUMMER, FALL} private final Season season; private static final List listWeather = new ArrayList (); private Weather (Season season) { this.season = season;} public Season getSeason () { return season;} static { for (Season season : Season.values()) { listWeather.add(new Weather(season)); } } public static ArrayList getWeatherList () { return listWeather; } public String toString(){ return season;} // takes advantage of toString() method of Season. }

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Q 17: What is the main difference between a String and a StringBuffer class? LF PI CI CO A 17: String

StringBuffer / StringBuilder

String is immutable: you can’t modify a string object but can replace it by creating a new instance. Creating a new instance is rather expensive.

StringBuffer is mutable: use StringBuffer or StringBuilder when you want to modify the contents. StringBuilder was added in Java 5 and it is identical in all respects to StringBuffer except that it is not synchronised, which makes it slightly faster at the cost of not being thread-safe.

//Inefficient version using immutable String String output = “Some text” Int count = 100; for(int I =0; i from the operating system and it responds to this signal by printing out a Java stack trace and then continue to run.

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The JVM continues to run because the JVM has a special built-in debug routine, which will trap the signal -3. On the other hand signals like SIGSTOP (kill -23 ) and SIGKILL (kill -9 ) will cause the JVM process to stop or die. The following JVM argument will indicate JVM not to pause on SIGQUIT signal from the operating system. Java –Xsqnopause

Java – Personal

Q 66: Did you have to use any design patterns in your Java project? DP A 66: Yes. Refer Q10 [Strategy], Q14 [Iterator], Q20 [Decorator], Q31 [Visitor], Q45 [Singleton], Q46 [Factory], Q50 [Command], and Q54 [MVC] in Java section and Q11 in How would you go about… section. Note: Learning of other patterns recommended (Gang of Four Design Patterns). Resource: http://www.patterndepot.com/put/8/JavaPatterns.htm. Why use design patterns, you may ask (Refer Q5 in Enterprise section). Design patterns are worthy of mention in your CV and interview. Design patterns have a number of advantages: ƒ ƒ ƒ ƒ

Capture design experience from the past. Promote reuse without having to reinvent the wheel. Define the system structure better. Provide a common design vocabulary.

Some advice if you are just starting on your design pattern journey: ƒ

If you are not familiar with UML, now is the time. UML is commonly used to describe patterns in pattern catalogues, including class diagrams, sequence diagrams etc. (Refer Q106 - Q109 in Enterprise section).

ƒ

When using patterns, it is important to define a naming convention. It will be much easier to manage a project as it grows to identify exactly what role an object plays with the help of a naming convention e.g. AccountFacilityBusinessDelegate, AccountFacilityFactory, AccountFacilityValueObject, AccountDecorator, AccountVisitor, AccountTransferObject (or AccountFacilityVO or AccountTO).

ƒ

Make a list of requirements that you will be addressing and then try to identify relevant patterns that are applicable.

Q 67: Tell me about yourself or about some of the recent projects you have worked with? What do you consider your most significant achievement? Why do you think you are qualified for this position? Why should we hire you and what kind of contributions will you make? A 67: [Hint:] Pick your recent projects and brief on it. Also is imperative that during your briefing, you demonstrate how you applied your skills and knowledge in some of the following areas: ƒ ƒ ƒ ƒ

Design concepts and design patterns: How you understand and applied them. Performance and memory issues: How you identified and fixed them. Exception handling and best practices: How you understand and applied them. Multi-threading and concurrent access: How you identified and fixed them.

Some of the questions in this section can help you prepare your answers by relating them to your current or past work experience. For example: ƒ ƒ ƒ ƒ ƒ ƒ

Design Concepts: Refer Q5, Q6, Q7, Q8, Q9 etc Design Patterns: Refer Q10, Q14, Q20, Q31, Q45, Q46, Q50 etc [Refer Q11 in How would you go about…? section] Performance issues: Refer Q21, Q63 etc Memory issues: Refer Q32, Q64, Q65 etc Exception Handling: Refer Q34, Q35 etc Multi-threading (Concurrency issues): Refer Q29, Q40 etc

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Demonstrating your knowledge in the above mentioned areas will improve your chances of being successful in your Java/J2EE interviews. 90% of the interview questions are asked based on your own resume. So in my view it is also very beneficial to mention how you demonstrated your knowledge/skills by stepping through a recent project on your resume. The two other areas, which I have not mentioned in this section, which are also very vital, are transactions and security. These two areas will be covered in the next section, which is the Enterprise section (J2EE, JDBC, EJB, JMS, SQL, XML etc). Even if you have not applied these skills knowingly or you have not applied them at all, just demonstrating that you have the knowledge and an appreciation will help you improve your chances in the interviews. Also mention any long hours worked to meet the deadline, working under pressure, fixing important issues like performance issues, running out of memory issues etc.

Q 68: Why are you leaving your current position? A 68: [Hint] ƒ ƒ ƒ

Do not criticize your previous employer or coworkers or sound too opportunistic. It is fine to mention a major problem like a buy out, budget constraints, merger or liquidation. You may also say that your chance to make a contribution is very low due to company wide changes or looking for a more challenging senior or designer role.

Q 69: What do you like and/or dislike most about your current and/or last position? A 69: [Hint] The interviewer is trying to find the compatibility with the open position. So Do not say anything like: ƒ ƒ

You dislike overtime. You dislike management or coworkers etc.

It is safe to say: ƒ ƒ ƒ

You like challenges. Opportunity to grow into design, architecture, performance tuning etc. You dislike frustrating situations like identifying a memory leak problem or a complex transactional or a concurrency issue. You want to get on top of it as soon as possible.

Q 70: How do you handle pressure? Do you like or dislike these situations? A 70: [Hint] These questions could mean that the open position is pressure-packed and may be out of control. Know what you are getting into. If you do perform well under stress then give a descriptive example. High achievers tend to perform well in pressure situations.

Q 71: What are your strengths and weaknesses? Can you describe a situation where you took initiative? Can you describe a situation where you applied your problem solving skills?

A 71: [Hint] Strengths: ƒ

Taking initiatives and being pro-active: You can illustrate how you took initiative to fix a transactional issue, a performance problem or a memory leak problem.

ƒ

Design skills: You can illustrate how you designed a particular application using OO concepts.

ƒ

Problem solving skills: Explain how you will break a complex problem into more manageable sub-sections and then apply brain storming and analytical skills to solve the complex problem. Illustrate how you went about identifying a scalability issue or a memory leak problem.

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ƒ

Communication skills: Illustrate that you can communicate effectively with all the team members, business analysts, users, testers, stake holders etc.

ƒ

Ability to work in a team environment as well as independently: Illustrate that you are technically sound to work independently as well as have the interpersonal skills to fit into any team environment.

ƒ

Hard working, honest, and conscientious etc are the adjectives to describe you.

Weaknesses: Select a trait and come up with a solution to overcome your weakness. Stay away from personal qualities and concentrate more on professional traits for example: ƒ

I pride myself on being an attention to detail guy but sometimes miss small details. So I am working on applying the 80/20 principle to manage time and details. Spend 80% of my effort and time on 20% of the tasks, which are critical and important to the task at hand.

ƒ

Some times when there is a technical issue or a problem I tend to work continuously until I fix it without having a break. But what I have noticed and am trying to practise is that taking a break away from the problem and thinking outside the square will assist you in identifying the root cause of the problem sooner.

Q 72: What are your career goals? Where do you see yourself in 5-10 years? A 72: [Hint] Be realistic. For example Next 2-3 years to become a senior developer or a team lead. Next 3-5 years to become a solution designer or an architect.

ƒ ƒ

Note: For Q66 – Q72 tailor your answers to the job. Also be prepared for questions like: ƒ

What was the last Java related book or article you read? [Hint] ƒ ƒ ƒ ƒ

ƒ

Which Java related website(s) do you use to keep your knowledge up to date? [Hint] ƒ ƒ ƒ ƒ ƒ ƒ ƒ ƒ

ƒ

Material rewards such as salary, perks, benefits etc naturally come into play but focus on your achievements or accomplishments than on rewards.

Do you have any role models in software development? [Hint] ƒ

ƒ

http://www.theserverside.com http://www.javaworld.com http://www-136.ibm.com/developerworks/Java http://www.precisejava.com http://www.allapplabs.com http://java.sun.com http://www.martinfowler.com http://www.ambysoft.com

What past accomplishments gave you satisfaction? What makes you want to work hard? [Hint] ƒ

ƒ

Mastering EJB by Ed Roman. EJB design patterns by Floyd Marinescu. Bitter Java by Bruce Tate. Thinking in Java by Bruce Eckel.

Scott W. Ambler, Martin Fowler, Ed Roman, Floyd Marinescu, Grady Booch etc.

Why do you want to work for us? (Research the company prior to the interview).

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Java – Key Points

ƒ

Java is an object oriented (OO) language, which has built in support for multi-threading, socket communication, automatic memory management (i.e. garbage collection) and also has better portability than other languages across operating systems.

ƒ

Java class loaders are hierarchical and use a delegation model. The classes loaded by a child class loader have visibility into classes loaded by its parents up the hierarchy but the reverse is not true.

ƒ

Java does not support multiple implementation inheritance but supports multiple interface inheritance.

ƒ

Polymorphism, inheritance and encapsulation are the 3 pillar of an object-oriented language.

ƒ

Code reuse can be achieved through either inheritance (“is a” relationship) or object composition (“has a” relationship). Favour object composition over inheritance.

ƒ

When using implementation inheritance, make sure that the subclasses depend only on the behaviour of the superclass, not the actual implementation. An abstract base class usually provides an implementation inheritance.

ƒ

Favour interface inheritance to implementation inheritance because it promotes the deign concept of coding to interface and reduces coupling. The interface inheritance can achieve code reuse through object composition.

ƒ

Design by contract specifies the obligations of a calling-method and called-method to each other using preconditions, post-conditions and class invariants.

ƒ

When using Java collection API, prefer using ArrayList or HashMap as opposed to Vector or Hashtable to avoid any synchronization overhead. The ArrayList or HashMap can be externally synchronized for concurrent access by multiple threads.

ƒ

Set the initial capacity of a collection appropriately and program in terms of interfaces as opposed to implementations.

ƒ

When providing a user defined key class for storing objects in HashMap, you should override equals(), and hashCode() methods from the Object class.

ƒ

String class is immutable and StringBuffer and StringBuilder classes are mutable. So it is more efficient to use a StringBuffer or a StringBuilder as opposed to a String in a computation intensive situations (ie. in for, while loops).

ƒ

Serialization is a process of writing an object to a file or a stream. Transient variables cannot be serialized.

ƒ

Java I/O performance can be improved by using buffering, minimising access to the underlying hard disk and operating systems. Use the NIO package for performance enhancing features like non-blocking I/O operation, buffers to hold data, and memory mapping of files.

ƒ

Each time an object is created in Java it goes into the area of memory known as heap. The primitive variables are allocated in the stack if they are local method variables and in the heap if they are class member variables.

ƒ

Threads share the heap spaces so it is not thread-safe and the threads have their own stack space, which is thread-safe.

ƒ

The garbage collection cannot be forced, but you can nicely ask the garbage collector to collect garbage.

ƒ

There two types of exceptions checked (ie compiler checked) and unchecked (Runtime Exceptions). It is not advisable to catch type Exception.

ƒ

A process is an execution of a program (e.g. JVM process) but a thread is a single execution sequence within the process.

ƒ

Threads can be created in Java by either extending the Thread class or implementing the Runnable interface.

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ƒ

In Java each object has a lock and a thread can acquire a lock by using the synchronized key word. The synchronization key word can be applied in method level (coarse-grained lock) or block level (fine-grained lock which offers better performance) of code.

ƒ

Threads can communicate with each other using wait(), notify(), and notifyAll() methods. This communication solves the consumer-producer problem.

ƒ

Sockets are communication channels, which facilitate inter-process communication.

ƒ

Swing uses the MVC paradigm to provide loose coupling and action architecture to implement a shared behaviour between two or more user interface components.

ƒ

Swing components should be accessed through an event-dispatching thread. There is a way to access the Swing event-dispatching thread from outside event-handling or drawing code, is using SwingUtilities’ invokeLater() and invokeAndWait() methods.

ƒ

A signed applet can become a trusted applet, which can work outside the sandbox.

ƒ

In Java typically memory leak occurs when an object of longer life cycle has a reference to objects of a short life cycle.

ƒ

You can improve performance in Java by : 1. 2. 3. 4. 5. 6. 7.

Pooling your valuable resources like threads, database and socket connections. Optimizing your I/O operations. Minimising network overheads, calls to Date, Calendar related classes, use of “casting” or runtime type checking like “instanceof” in frequently executed methods/loops, JNI calls, etc Managing your objects efficiently by caching or recycling them without having to rely on garbage collection. Using a StringBuffer as opposed to String and ArrayList or HashMap as oppose to Vector or Hashtable Applying multi-threading where applicable. Minimise any potential memory leaks.

Finally, very briefly familiarise yourself with some of the key design patterns like:

ƒ

1. 2. 3. 4. 5. 6. 7. 8. 9.

Decorator design pattern: used by Java I/O API. A popular design pattern. Reactor design pattern/Observer design pattern: used by Java NIO API. Visitor design pattern: to avoid instanceof and typecast constructs. Factory method/abstract factory design pattern: popular pattern, which gets frequently asked in interviews. Singleton pattern: popular pattern, which gets frequently asked in interviews. Composite design pattern: used by GUI components and also a popular design pattern MVC design pattern/architecture: used by Swing components and also a popular pattern. Command pattern: used by Swing action architecture and also a popular design pattern. Strategy design pattern: A popular design pattern used by AWT layout managers.

Refer Q11 in “How would you go about…” section for a detailed discussion and code samples on GOF (Gang of Four) design patterns. Recommended reading on design patterns: ƒ

The famous Gang of Four book: Design Patterns, Eric Gamma, Richard Helm, Ralph Johnson, and John Vlissides (Addiso-Wesley Publishing Co., 1995; ISBN: 0201633612).

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Tips: ƒ

Try to find out the needs of the project in which you will be working and the needs of the people within the project.

ƒ

80% of the interview questions are based on your own resume.

ƒ

Where possible briefly demonstrate how you applied your skills/knowledge in the key areas [design concepts, transactional issues, performance issues, memory leaks etc] as described in this book. Find the right time to raise questions and answer those questions to show your strength.

ƒ

Be honest to answer technical questions, you are not expected to remember everything (for example you might know a few design patterns but not all of them etc). If you have not used a design pattern in question, request the interviewer, if you could describe a different design pattern.

ƒ

Do not be critical, focus on what you can do. Also try to be humorous to show your smartness.

ƒ

Do not act superior.

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SECTION TWO

Enterprise Java – Interview questions & answers

K E Y A R E A S

ƒ ƒ ƒ ƒ ƒ ƒ ƒ ƒ ƒ ƒ ƒ ƒ

Specification Fundamentals Design Concepts DC Design Patterns DP Concurrency Issues CI Performance Issues PI Memory Issues MI Exception Handling EH Transactional Issues TI Security SE Scalability Issues SI Best Practices BP Coding1 CO

SF

Popular Questions: Q02, Q03, Q10, Q16, Q19, Q20, Q24, Q25, Q30, Q31, Q36, Q39, Q40, Q45, Q46, Q48, Q49, Q53, Q58, Q63, Q64, Q65, Q66, Q71, Q72, Q73, Q76, Q77, Q78, Q79, Q83, Q84, Q85, Q86, Q87, Q89, Q90, Q91, Q93, Q96, Q97, Q98, Q100, Q102, Q106, Q107, Q110, Q123, Q124, Q125, Q129, Q131, Q136.

1

Unlike other key areas, the CO is not always shown against the question but shown above the actual subsection of relevance within a question.

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Enterprise - J2EE

Q 01: What is J2EE? What are J2EE components and services? SF A 01: J2EE (Java 2 Enterprise Edition) is an environment for developing and deploying enterprise applications. The J2EE platform consists of J2EE components, services, Application Programming Interfaces (APIs) and protocols that provide the functionality for developing multi-tiered and distributed Web based applications. J 2 E E P h y s ic a l T ie r s , C o n ta in e r s , C o m p o n e n ts , S e r v ic e s & F ir e w a ll

DMZ

A P Is

F ir e w a ll

in t e r n e t

C li e n t

W e b S e rv e r

D a ta b a s e S e rv e r

A p p li c a t i o n S e r v e r

A p p li c a t i o n T i e r ( M i d d l e T i e r )

A p p le t

H T T P (S )

W eb S e rv e r

J 2 E E A p p lic a tio n S e r v e r

W e b C o n ta in e r

JD BC

RDBM S

Tag lib r a r y

HTM L

RMI

JDBC RMI / IIOP

JTA

RMI/IIOP

CSS

J a v a M a il

JS P

JNDI

S e r v le t s

JAF

H T T P (S )

JavaMail

(X )H T M L , XM L (B ro w s e r)

D a ta (E IS ) T ie r

JMS

C l i e n t T ie r

JM S

Java A p p lic a t io n

M e s s a g in g

E J B C o n ta in e r R M I/IIO P

IIO P

C o rb a S e rv e r

JAF

JavaMail

M e s s a g e d r iv e n b e a n s

JMS

E n t it y B e a n s

JDBC

JNDI

RMI/IIOP

S e s s io n B e a n s

JTA

C lie n t A p p lic a t io n ( s t a n d a lo n e J a v a p ro g ra m )

JN D I

D ir e c t o r y S e r v ic e

O th e r S e rv ic e s + A P I s p ro v id e d b y s e rv e r/ c o n ta in e r: S e c u r it y ( S S L , A C L , J A A S , X .5 0 9 ) t r a n s a c t io n s , t h r e a d in g , R e s o u r c e p o o lin g ( E g : C o n n e c t io n p o o lin g ) e t c ,F a u lt T o le r a n c e , L o a d B a la n c in g , c lu s t e r in g M o n it o r in g , A u d it in g , L o g g in g e t c m o r e ...... ..... ....

A J2EE component is a self-contained functional software unit that is assembled into a J2EE application with its related classes and files and communicates with other components. The J2EE specification defines the following J2EE components: Component type Applet Application client Web component Enterprise JavaBeans Enterprise application Resource adapters

Components applets Client side Java codes. JSP, Servlet Session beans, Entity beans, Message driven beans WAR, JAR, etc Resource adapters

Packaged as JAR (Java ARchive) JAR (Java ARchive) WAR (Web ARchive) JAR (EJB Archive) EAR (Enterprise ARchive) RAR (Resource Adapter ARchive)

So what is the difference between a component and a service you may ask? A component is an application level software unit as shown in the table above. All the J2EE components depend on the container for the system level support like transactions, security, pooling, life cycle management, threading etc. A service is a component

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that can be used remotely through a remote interface either synchronously or asynchronously (e.g. Web service, messaging system, sockets, RPC etc). Containers (Web & EJB containers) are the interface between a J2EE component and the low level platform specific functionality that supports J2EE components. Before a Web, enterprise bean (EJB), or application client component can be executed, it must be assembled into a J2EE module (jar, war, and/or ear) and deployed into its container. A J2EE server provides system level support services such us security, transaction management, JNDI (Java Naming and Directory Interface) lookups, remote access etc. J2EE architecture provides configurable and nonconfigurable services. The configurable service enables the J2EE components within the same J2EE application to behave differently based on where they are deployed. For example the security settings can be different for the same J2EE application in two different production environments. The non-configurable services include enterprise bean (EJB) and servlet life cycle management, resource pooling etc. Protocols are used for access to Internet services. J2EE platform supports HTTP (HyperText Transfer Protocol), TCP/IP (Transmission Control Protocol / Internet Protocol), RMI (Remote Method Invocation), SOAP (Simple Object Access Protocol) and SSL (Secured Socket Layer) protocol. The J2EE API can be summarised as follows: J2EE technology category Component model technology Web services technology Other

API (Application Program Interface) Java Servlet, JavaServer Pages(JSP), Enterprise JavaBeans(EJB). JAXP (Java API for XML Processing), JAXR (Java API for XML Registries), SAAJ (SOAP with attachment API for Java), JAX-RPC (Java API for XML-based RPC), JAX-WS (Java API for XML-based Web Services). JDBC (Java Database Connectivity), JNDI (Java Naming and Directory Interface), JMS (Java Messaging Service), JCA (J2EE Connector Architecture), JTA (Java Transaction API), JavaMail, JAF (JavaBeans Activation Framework – used by JavaMail), JAAS (Java Authentication and Authorization Service), JMX (Java Management eXtenstions).

Q 02: Explain the J2EE 3-tier or n-tier architecture? SF DC A 02: This is a very commonly asked question. Be prepared to draw some diagrams on the board. The J2EE platform is a multi-tiered system. A tier is a logical or functional partitioning of a system. 2 – tier system

3 – tier system

2 -T ie r (C lie n t/S e rv e r) C lie n t M /C 1 U s e rIn te rfa c e /d is p la y L o g ic B u s in e s s lo g ic D a ta b a s e lo g ic

3 -T ie r (o r n -tie r)

C lie n t M /C 2 U s e rIn te rfa c e /d is p la y L o g ic B u s in e s s lo g ic D a ta b a s e lo g ic

C lie n t M /C 1 U s e rIn te rfa c e /d is p la y lo g ic

C lie n t M /C 2 U s e rIn te rfa c e /d is p la y lo g ic

M id d le -tie r s e rve r B u s in e ss L o g ic D a ta b as e L o g ic

B u s in e s s L o g ic D a ta b a s e lo g ic D a ta

D a ta

D a ta b a s e

When the developers are not disciplined, The display logic, business logic and database logic are muddled up and/or duplicated in a 2tier client server system.

D a tab a s e

The advantages of the multi-tier architecture are: ƒ ƒ ƒ

Forced separation of user interface logic and business logic. Business logic sits on small number of centralized machines (may be just one). Easy to maintain, to manage, to scale, loosely coupled etc.

Each tier is assigned a unique responsibility in a 3-tier system. Each tier is logically separated and loosely coupled from each other, and may be distributed.

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Client tier represents Web browser, a Java or other application, Applet, WAP phone etc. The client tier makes requests to the Web server who will be serving the request by either returning static content if it is present in the Web server or forwards the request to either Servlet or JSP in the application server for either static or dynamic content. Presentation tier encapsulates the presentation logic required to serve clients. A Servlet or JSP in the presentation tier intercepts client requests, manages logons, sessions, accesses the business services, and finally constructs a response, which gets delivered to client. Business tier provides the business services. This tier contains the business logic and the business data. All the business logic is centralised into this tier as opposed to 2-tier systems where the business logic is scattered between the front end and the backend. The benefit of having a centralised business tier is that same business logic can support different types of clients like browser, WAP, other stand-alone applications etc. Integration tier is responsible for communicating with external resources such as databases, legacy systems, ERP systems, messaging systems like MQSeries etc. The components in this tier use JDBC, JMS, J2EE Connector Architecture (JCA) and some proprietary middleware to access the resource tier. Resource tier is the external resource such as a database, ERP system, Mainframe system etc responsible for storing the data. This tier is also known as Data Tier or EIS (Enterprise Information System) Tier. J 2 E E T ie r s H ig h L e v e l T ie r s

L o g ic a l o r F u n c tio n a l T ie r s

C lie n t T ie r

C lie n t

C lie n t T ie r A p p le ts , H T M L ,W M L , J a v a S c r ip t , A p p lic a t io n C lie n t s e t c

M id d le T ie r P r e s e n ta tio n T ie r H T M L , C S S , G I F F ile s e t c ( s ta tic c o n t e n t) J S P , S e r v le t s , T a g lib r a r y a n d o th e r U I e le m e n ts ( C S S , G IF , e tc ) B u s in e s s T ie r E J B , J a v a C la s s e s , B u s in e s s O b je c t s e tc In te g r a tio n T ie r J M S , J D B C , C o n n e c to rs (J C A ), and Legacy

ly J2EE patterns app

W e b S e rv e r

A p p lic a tio n S e r v e r

D a ta T ie r

XM L

R e s o u r c e T ie r D a ta b a s e s , E R P & C R M s y s te m s , L e g a c y S y s te m s e tc

R D BM S

Note: On a high level J2EE can be construed as a 3-tier system consisting of Client Tier, Middle Tier (or Application Tier) and Data Tier. But logically or functionally J2EE is a multi-tier (or n-tier) platform. The advantages of a 3-tiered or n-tiered application: 3-tier or multi-tier architectures force separation among presentation logic, business logic and database logic. Let us look at some of the key benefits: ƒ

Manageability: Each tier can be monitored, tuned and upgraded independently and different people can have clearly defined responsibilities.

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ƒ

Scalability: More hardware can be added and allows clustering (i.e. horizontal scaling).

ƒ

Maintainability: Changes and upgrades can be performed without affecting other components.

ƒ

Availability: Clustering and load balancing can provide availability.

ƒ

Extensibility: Additional features can be easily added.

Q 03: Explain MVC architecture relating to J2EE? DC DP A 03: This is also a very popular interview question. MVC stands for Model-View-Controller architecture. It divides the functionality of displaying and maintaining of the data to minimise the degree of coupling (i.e. promotes loose coupling) between components.

J 2 E E M V C (M o d e l-V ie w -C o n tro lle r)

ge an ch

V ie w (J S P , J a v a B e a n s , S W IN G , C u s to m T a g s , e tc ) ¾ R e n d e rs th e m o d e l & h a s o n ly d is p la y lo g ic . ¾ S e n d s u s e r a c tio n s to th e c o n tro lle r ¾ A llo w s c o n tro lle r to s e le c t a v ie w .

e at st

ge tD ata to d isp lay

M odel (E n titiy B e a n s (E J B ), S e s s io n B e a n s (E J B ), P la in J a v a C la s s e s ) ¾ E n c a p s u la te s b u s in e s s lo g ic a n d a p p lic a tio n s ta te .

U s e r A c tio n (e g : s u b m ittin g a fo rm , c lic k in g a b u tto n e tc )

V ie w s e le c tio n ( e g :

¾ ¾ ¾ ¾

C o n tro lle r

1. Request

S e rv le t

2.

4.redire ct

Browser 6. R esponse

C lie n t T ie r

s e le c tin g th e n e xt

JS P p a g e to d isp la y a s a re s p o n se )

C o n tro lle r (S e rv le t, S tru ts A c tio n e tc ) c o n tro ls a p p lic a tio n b e h a v io u r M a p s u s e r a c tio n s to m o d e l. s e le c ts v ie w fo r re s p o n s e . u s u a lly o n e fo r e a c h fu n c tio n a lity .

V ie w JS P

in

st

an t

ia te

s M odel

5

E J B o r P la in J a v a c la s s

M id d le T ie r (A p p lic a tio n T ie r)

3 . re a d /u p d a te d a ta

d a ta b a s e

D a ta T ie r

N o te : T yp ical M V C arch itectu re is sh o w n ab o v e. V ariatio n s are p o ssib le (eg: M o d el 1 v s M o d el 2 M VC)

A model represents the core business logic and state. A model commonly maps to data in the database and will also contain core business logic. A View renders the contents of a model. A view accesses the data from the model and adds display logic to present the data. A Controller acts as the glue between a model and a view. A controller delegates the request to the model for application logic and state and also centralises the logic for dispatching the request to the next view based on the input parameters from the client and the application state. A controller also decouples JSP pages and the Servlet by handling the view selection.

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Q 04: How to package a module, which is, shared by both the WEB and the EJB modules? SF A 04: Package the modules shared by both WEB and EJB modules as dependency jar files. Define the Class-Path: property in the MANIFEST.MF file in the EJB jar and the Web war files to refer to the shared modules. [Refer Q7 in Enterprise section for diagram: J2EE deployment structure]. The MANIFEST.MF files in the EJB jar and WEB war modules should look like: Manifest-Version: 1.0 Created-By: Apache Ant 1.5 Class-Path: myAppsUtil.jar

Q 05: Why use design patterns in a J2EE application? DP A 05: ƒ

They have been proven. Patterns reflect the experience and knowledge of developers who have successfully used these patterns in their own work. It lets you leverage the collective experience of the development community. Example Session facade and value object patterns evolved from performance problems experienced due to multiple network calls to the EJB tier from the WEB tier. Fast lane reader and Data Access Object patterns exist for improving database access performance. The flyweight pattern improves application performance through object reuse (which minimises the overhead such as memory allocation, garbage collection etc).

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They provide common vocabulary. Patterns provide software designers with a common vocabulary. Ideas can be conveyed to developers using this common vocabulary and format. Example Should we use a Data Access Object (DAO)? How about using a Business Delegate? Should we use Value Objects to reduce network overhead? Etc.

Q 06: What is the difference between a Web server and an application server? SF A 06: Web Server

Application Server

Supports HTTP protocol. When the Web server receives an HTTP request, it responds with an HTTP response, such as sending back an HTML page (static content) or delegates the dynamic response generation to some other program such as CGI scripts or Servlets or JSPs in the application server. Uses various scalability and fault-tolerance techniques.

Exposes business logic and dynamic content to the client through various protocols such as HTTP, TCP/IP, IIOP, JRMP etc.

Uses various scalability and fault-tolerance techniques. In addition provides resource pooling, component life cycle management, transaction management, messaging, security etc. Provides services for components like Web container for servlet components and EJB container for EJB components.

With the advent of XML Web services the line between application servers and Web servers is not clear-cut. By passing XML documents between request and response the Web server can behave like an application server.

Q 07: What are ear, war and jar files? What are J2EE Deployment Descriptors? SF A 07: ear, war and jar are standard application deployment archive files. Since they are a standard, any application server (at least in theory) will know how to unpack and deploy them. An EAR file is a standard JAR file with an “.ear” extension, named from Enterprise ARchive file. A J2EE application with all of its modules is delivered in EAR file. JAR files can’t have other JAR files. But EAR and WAR (Web ARchive) files can have JAR files. An EAR file contains all the JARs and WARs belonging to an application. JAR files contain the EJB classes and WAR files contain the Web components (JSPs, static content (HTML, CSS, GIF etc), Servlets etc.). The J2EE application client's class files are also stored in a JAR file. EARs, JARs, and WARs all contain an XML-based deployment descriptor.

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J2EE deployment structure (ear, war, jar )

MyApps.ear MANIFEST.MF Manifest-Version: 1.0 Craeted-By: Apache Ant

META-INF application.xml deployment descriptor class files, properties files,configuration files etc

log4j.jar (3rd party jars) MyAppsCommon.jar , MyAppsUtil.jar (shared by both EJB and Web modules)

MyAppsEJB.jar

class files, properties files,configuration files etc

MANIFEST.MF class-path: log4j.jar MyAppsCommon.jar MyAppsUtil.jar

META-INF

ejb-jar.xml deployment descriptor

ejb classes , non-ejb class etc

MyAppsWeb.war

public directory (document root)

JSP, HTML, CSS, GIF (can have sub-folders) MANIFEST.MF class-path: log4j.jar MyAppsCommon.jar MyAppsUtil.jar

META-INF

web.xml deployment descriptor

WEB-INF private directory

lib

struts.jar, crimson.jar 3rd party jar files

classes

class files

Deployment Descriptors A deployment descriptor is an XML based text file with a “.xml” extension that describes a component's deployment settings. A J2EE application and each of its modules has its own deployment descriptor. Pay attention to elements marked in bold in the sample deployment descriptor files shown below. ƒ

application.xml: is a standard J2EE deployment descriptor, which includes the following structural information: EJB jar modules, WEB war modules, etc. Also since EJB jar modules are packaged as jars the same way dependency libraries like log4j.jar, commonUtil.jar etc are packaged, the application.xml descriptor will distinguish between these two jar files by explicitly specifying the EJB jar modules. MyApps MyAppsEJB.jar

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MyAppsWeb.war myAppsWeb Management position managger

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ejb-jar.xml: is a standard deployment descriptor for an EJB module. MyAppsEJB ContentService ejb.ContentServiceHome ejb.ContentService ejb.ContentServiceBean Stateless Bean Bid ejb.BidHome ejb.Bid ejb.BidBean Container ejb.BidPK False bid bidder bidDate id Employee is allowed to ... employee employee ContentService * Bid findByPrimaryKey

Enterprise Java Bid * Required

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web.xml: is a standard deployment descriptor for a WEB module. myWebApplication GlobalContext.ClassName web.GlobalContext MyWebController web.MyWebController config /WEB-INF/config/myConfig.xml 1 MyWebController /execute/* 400 /WEB-INF/jsp/errors/myError.jsp /WEB-INF/struts-bean.tld /WEB-INF/lib/taglib/struts/struts-bean.tld Employer /execute/employ POST GET PUT advisor FORM FBA /execute/MyLogon

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/execute/MyError Advisor advisor

Q 08: Explain J2EE class loaders? SF A 08: J2EE application server sample class loader hierarchy is shown below. (Also refer to Q4 in Java section). As per the diagram the J2EE application specific class loaders are children of the “System –classpath” class loader. When the parent class loader is above the “System –Classpath” class loader in the hierarchy as shown in the diagram (i.e. bootstrap class loader or extensions class loader) then child class loaders implicitly have visibility to the classes loaded by its parents. When a parent class loader is below a “System -Classpath” class loader then the child class loaders will only have visibility into the classes loaded by its parents only if they are explicitly specified in a manifest file (MANIFEST.MF) of the child class loader. Example As per the diagram, if the EJB module MyAppsEJB.jar wants to refer to MyAppsCommon.jar and MyAppsUtil.jar we need to add the following entry in the MyAppsEJB.jar’s manifest file MANIFEST.MF. class-path: MyAppsCommon.jar MyAppsUtil.jar J 2 E E a p p l ic a ti o n s e r v e r s a m p le c l a s s lo a d e r h ie r a r c h y M y A p p s .e a r

B oo tstr ap (JV M ) (r t.j ar , i1 8 .ja r )

M y A p p s U t il .ja r

E x te n sio n s(J V M ) (li b/ e xt)

M y A p p s C o m m o n .j a r M y A p p s E J B . ja r

S y stem (J V M ) (- cl as sp ath)

M yA p p s W e b .w a r A pp li ca tio n c la ss lo ad e r (E A R )

A p p lic atio n cl as s l oa d e r (E A R )

E J B c la ss lo a d e r

E J B c la ss l oa d er

W A R c la ss lo ad e r

W A R cl as s l oa d e r

W A R c las s lo ad e r

E a c h E A R g e ts its o w n in s ta n c e o f c la s s lo a d e r A ll th e E J B ja rs in a e a r file s h a re th e s a m e E J B c la s s lo a d e r. E a c h W A R g e ts its o w n in s ta n c e o f c la s s lo a d e r. T h e W E B -IN F /lib lib ra rie s a re s p e c ific to e a c h W A R

N o t e : A p p lic a t io n v e n d o r's S e r v e r c la s s lo a d e r h ie r a rc h y m ig h t s lig h tly v a r y .

This is because the application (EAR) class loader loads the MyAppsCommon.jar and MyAppsUtil.jar. The EJB class loader loads the MyAppsEJB.jar, which is the child class loader of the application class loader. The WAR class loader loads the MyAppsWeb.war. Every J2EE application or EAR gets its own instance of the application class loader. This class loader is responsible for loading all the dependency jar files, which are shared by both WEB and EJB modules. For example third party libraries like log4j, utility classes, shared classes or common classes (Exception thrown by an EJB module should be caught by a WEB module) etc. The key difference between the EJB and WAR class loader is that all the EJB jars in the application share the same EJB class loader whereas WAR files get their own class loader. This is because the EJBs have inherent relationship between one another (ie EJB-EJB communication between EJBs in different applications but hosted on the same JVM) but the Web modules do not. Every WAR file should be able to have its own WEB-INF/lib third party libraries and need to be able to load its own version of converted logon.jsp Servlet so each WEB module is isolated in its own class loader.

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So if two different WEB modules want to use two different versions of the same EJB then we need to have two different ear files. As was discussed in the Q4 in Java section the class loaders use a delegation model where the child class loaders delegate the loading up the hierarchy to their parent before trying to load it itself only if the parent can’t load it. But with regards to WAR class loaders, some application servers provide a setting to turn this behaviour off (DelegationMode=false). This delegation mode is recommended in the Servlet 2.3 specification. As a general rule classes should not be deployed higher in the hierarchy than they are supposed to exist. This is because if you move one class up the hierarchy then you will have to move other classes up the hierarchy as well. This is because classes loaded by the parent class loader can’t see the classes loaded by its child class loaders (uni-directional bottom-up visibility).

Enterprise - Servlet

Q 09: What is the difference between CGI and Servlet? SF Q 09: Traditional CGI (Common Gateway Interface) Traditional CGI creates a heavy weight process to handle each http request. N number of copies of the same traditional CGI programs is copied into memory to serve N number of requests.

Java Servlet Spawns a lightweight Java thread to handle each http request. Single copy of a type of servlet but N number of threads (thread sizes can be configured in an application server).

In the Model 2 MVC architecture, servlets process requests and select JSP views. So servlets act as controller. Servlets intercept the incoming HTTP requests from the client (browser) and then dispatch the request to the business logic model (e.g. EJB, POJO - Plain Old Java Object, JavaBeans etc). Then select the next JSP view for display and deliver the view to client as the presentation (response). It is the best practice to use Web tier UI frameworks like Struts, JavaServer Faces etc, which uses proven and tested design patterns.

Q 10: HTTP is a stateless protocol, so how do you maintain state? How do you store user data between requests? SF PI BP

A 10: This is a commonly asked question as well. You can retain the state information between different page requests as follows: HTTP Sessions are the recommended approach. A session identifies the requests that originate from the same browser during the period of conversation. All the servlets can share the same session. The JSESSIONID is generated by the server and can be passed to client through cookies, URL re-writing (if cookies are turned off) or built-in SSL mechanism. Care should be taken to minimize size of objects stored in session and objects stored in session should be serializable. In a Java servlet the session can be obtained as follows: CO HttpSession session = request.getSession(); //returns current session or a new session

Sessions can be timed out (configured in web.xml) or manually invalidated.

Session Management Server

Client (Browser) 1. Initial Request[No session]

2. JSESSIONID is passed to client with the response through cookies or URL re-writing 3. Client uses the JSESSIONID for subsequent requests

A new session is created on the Server side with JSESSIONID where state can be maintained as name/value pair. for the rmation tate info IONID s d re sto SS retrieve supplied JSE

JSESSIONID

Name

Value

xsder12345

Firstname

Peter

xsder12345

LastName

Smith

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Hidden Fields on the pages can maintain state and they are not visible on the browser. The server treats both hidden and non-hidden fields the same way.

The disadvantage of hidden fields is that they may expose sensitive or private information to others. URL re-writing will append the state information as a query string to the URL. This should not be used to maintain private or sensitive information. Http://MyServer:8080/MyServlet?Firstname=Peter&Lastname=Smith

Cookies: A cookie is a piece of text that a Web server can store on a user’s hard disk. Cookies allow a website to store information on a user’s machine and later retrieve it. These pieces of information are stored as name-value pairs. The cookie data moves in the following manner: ™

If you type the URL of a website into your browser, your browser sends the request to the Web server. When the browser does this it looks on your machine for a cookie file that URL has set. If it finds it, your browser will send all of the name-value pairs along with the URL. If it does not find a cookie file, it sends no cookie data.

™

The URL’s Web server receives the cookie data and requests for a page. If name-value pairs are received, the server can use them. If no name-value pairs are received, the server can create a new ID and then sends name-value pairs to your machine in the header for the Web page it sends. Your machine stores the name value pairs on your hard disk.

Cookies can be used to determine how many visitors visit your site. It can also determine how many are new versus repeated visitors. The way it does this is by using a database. The first time a visitor arrives, the site creates a new ID in the database and sends the ID as a cookie. The next time the same user comes back, the site can increment a counter associated with that ID in the database and know how many times that visitor returns. The sites can also store user preferences so that site can look different for each visitor. Which mechanism to choose? Session Description mechanism HttpSession

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There is no limit on the size of the session data kept. The performance is good. This is the preferred way of maintaining state. If we use the HTTP session with the application server’s persistence mechanism (server converts the session object into BLOB type and stores it in the Database) then the performance will be moderate to poor.

Note: When using HttpSession mechanism you need to take care of the following points: ƒ ƒ ƒ

Hidden fields URL rewriting Cookies

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Remove session explicitly when you no longer require it. Set the session timeout value. Your application server may serialize session objects after crossing a certain memory limit. This is expensive and affects performance. So decide carefully what you want to store in a session. There is no limit on size of the session data. May expose sensitive or private information to others (So not good for sensitive information). The performance is moderate. There is a limit on the size of the session data. Should not be used for sensitive or private information. The performance is moderate. There is a limit for cookie size. The browser may turn off cookies. The performance is moderate.

The benefit of the cookies is that state information can be stored regardless of which server the client talks to and even if all servers go down. Also, if required, state information can be retained across sessions.

Q 11: Explain the life cycle methods of a servlet? SF A 11: The Web container is responsible for managing the servlet’s life cycle. The Web container creates an instance of the servlet and then the container calls the init() method. At the completion of the init() method the servlet is in

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ready state to service requests from clients. The container calls the servlet’s service() method for handling each request by spawning a new thread for each request from the Web container’s thread pool [It is also possible to have a single threaded Servlet, refer Q16 in Enterprise section]. Before destroying the instance the container will call the destroy() method. After destroy() the servlet becomes the potential candidate for garbage collection. Note on servlet reloading: Most servers can reload a servlet after its class file has been modified provided the servlets are deployed to $server_root/servlets directory. This is achieved with the help of a custom class loader. This feature is handy for development and test phases. This is not recommended for production since it can degrade performance because of timestamp comparison for each request to determine if a class file has changed. So for production it is recommended to move the servlet to server’s class path ie $server_root/classes. When a server dispatches a request to a servlet, the server first checks if the servlet's class file has changed on disk. If it has changed, the server abandons the class loader used to load the old version and creates a new instance of the custom class loader to load the new version. Old servlet versions can stay in memory indefinitely (so the effect is the other classes can still hold references to the old servlet instances, causing odd side effects, but the old versions are not used to handle any more requests. Servlet reloading is not performed for classes found in the server's classpath because the core, primordial class loader, loads those classes. These classes are loaded once and retained in memory even when their class files change.

S e rvlet L ife C yc le instantiate & call init()

ca lle d o n ce

init() ready to serve requests

thread 1 : client request thread 2 : client request thread 3 : client request

service()

ca lle d o n ce

handle m ultiple requests and send response.

destroy()

Q 12: Explain the directory structure of a WEB application? SF SE A 12: Refer Q7 in Enterprise section for diagram: J2EE deployment structure and explanation in this section where MyAppsWeb.war is depicting the Web application directory structure. The directory structure of a Web application consists of two parts: ƒ

A public resource directory (document root): The document root is where JSP pages, client-side classes and archives, and static Web resources are stored.

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A private directory called WEB-INF: which contains following files and directories: ƒ ƒ ƒ ƒ

web.xml : Web application deployment descriptor. *.tld : Tag library descriptor files. classes : A directory that contains server side classes like servlets, utility classes, JavaBeans etc. lib : A directory where JAR (archive files of tag libraries, utility libraries used by the server side classes) files are stored.

Note: JSP resources usually reside directly or under subdirectories of the document root, which are directly accessible to the user through the URL. If you want to protect your Web resources then hiding the JSP files behind the WEB-INF directory can protect the JSP files from direct access. Refer Q35 in Enterprise section.

Q 13: What is the difference between doGet () and doPost () or GET and POST? SF SE

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POST or doPost()

The request parameters are transmitted as a query string appended to the request. Allows browser bookmarks but not appropriate for transmitting private or sensitive information. http://MyServer/MyServlet?name=paul This is a security risk. GET was originally intended for static resource retrieval.

The request parameters are passed with the body of the request. More secured. POST was intended for input data submits where the results are expected to change.

GET is not appropriate when large amounts of input data are being transferred.

Q 14: What are the ServletContext and ServletConfig objects? What are Servlet environment objects? SF A 14: The Servlet Engine uses both objects. ServletConfig

ServletContext

The ServletConfig parameters are for a particular Servlet. The parameters are specified in the web.xml (ie deployment descriptor).

The ServletContext parameters are specified for the entire Web application. The parameters are specified in the web.xml (ie deployment descriptor).

Servlet Environment Objects Servlet Engine Web App1 init param for the web app

web.xml (Deployment Descriptor) snippets ServletContext

GlobalClassName MyWebAppClass

MyServlet1

config

MyServlet2

config ServletConfig objects init params set for eachSevlet

Web App2



............ ........

MyServlet1 com.MyServlet config /WEB-INF/config/config.xml 1



Q 15: What is the difference between HttpServlet and GenericServlet? SF A 15: GenericServlet

HttpServlet

A GenericServlet has a service() method to handle requests.

The HttpServlet extends GenericServlet and adds support for HTTP protocol based methods like doGet(), doPost(), doHead() etc.

Protocol independent.

Protocol dependent.

Q 16: How do you make a Servlet thread safe? What do you need to be concerned about with storing data in Servlet instance fields? CI PI BP

A 16: As shown in the figure Servlet Life Cycle in Q11 in Enterprise section, a typical (or default) Servlet life cycle creates a single instance of each servlet and creates multiple threads to handle the service() method. The multithreading aids efficiency but the servlet code must be coded in a thread safe manner. The shared resources (e.g. instance variables, utility or helper objects etc) should be appropriately synchronized or should only use variables in a read-only manner. Having large chunks of code in synchronized blocks in your service methods can adversely affect performance and makes the code more complex.

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Alternatively it is possible to have a single threaded model of a servlet by implementing the marker or null interface javax.servlet.SingleThreadedModel. The container will use one of the following approaches to ensure thread safety: ƒ ƒ

Instance pooling where container maintains a pool of servlets. Sequential processing where new requests will wait while the current request is being processed.

Best practice: It is best practice to use multi-threading and stay away from the single threaded model of the servlet unless otherwise there is a compelling reason for it. Shared resources can be synchronized or used in read-only manner or shared values can be stored in a database table. The single threaded model can adversely affect performance.

Q 17: What is pre-initialization of a Servlet? LF A 17: By default the container does not initialize the servlets as soon as it starts up. It initializes a servlet when it receives a request for the first time for that servlet. This is called lazy loading. The servlet deployment descriptor (web.xml) defines the element, which can be configured to make the servlet container load and initialize the servlet as soon as it starts up. The process of loading a servlet before any request comes in is called pre-loading or pre-initializing a servlet. We can also specify the order in which the servlets are initialized.

2

Q 18: What is a RequestDispatcher? What object do you use to forward a request? LF CO A 18: A Servlet can obtain its RequestDispatcher object from its ServletContext. //…inside the doGet() method ServletContext sc = getServletContext(); RequestDispatcher rd = sc.getRequestDispatcher(url); // forwards the control to another servlet or JSP to generate response. This method allows one servlet to do preliminary //processing of a request and another resource to generate the response rd.forward(request,response); or // includes the content of the resource such as Servlet, JSP, HTML, Images etc into the calling Servlet’s response. rd.include(request, response);

Q 19: What is the difference between forwarding a request and redirecting a request? LF DC A 19: Both methods redirect you to a new resource like Servlet, JSP etc. But redirecting - sendRedirect()

forward

Sends a header back to the browser, which contains the name of the resource to be redirected to. The browser will make a fresh request from this header information. Need to provide absolute URL path. Has an overhead of extra remote trip but has the advantage of being able to refer to any resource on the same or different domain and also allows book marking of the page.

Forward action takes place within the server without the knowledge of the browser. No extra network trip.

Q 20: What are the considerations for servlet clustering? DC SI A 20: The clustering promotes high availability and scalability. The considerations for servlet clustering are: ƒ ƒ ƒ ƒ ƒ

Objects stored in a session should be serializable to support in-memory replication of sessions. Also consider the overhead of serializing very large objects. Test the performance to make sure it is acceptable. Design for idempotence. Failure of a request or impatient users clicking again can result in duplicate requests being submitted. So the Servlets should be able to tolerate duplicate requests. Avoid using instance and static variables in read and write mode because different instances may exist on different JVMs. Any state should be held in an external resource such as a database. Avoid storing values in a ServletContext. A ServletContext is not serializable and also the different instances may exist in different JVMs. Avoid using java.io.* because the files may not exist on all backend machines. Instead use getResourceAsStream().

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Q 21: If an object is stored in a session and subsequently you change the state of the object, will this state change replicated to all the other distributed sessions in the cluster? DC SI

A 21: No. Session replication is the term that is used when your current service state is being replicated across multiple application instances. Session replication occurs when we replicate the information (ie session attributes) that are stored in your HttpSession. The container propagates the changes only when you call the setAttribute(……) method. So mutating the objects in a session and then by-passing the setAttribute(………..) will not replicate the state change. CO Example If you have an ArrayList in the session representing shopping cart objects and if you just call getAttribute() to retrieve the ArrayList and then add or change something without calling the setAttribute(……………) then the container may not know that you have added or changed something in the ArrayList. So the session will not be replicated.

Q 22: What is a filter, and how does it work? LF DP A 22: A filter dynamically intercepts requests and responses to transform or use the information contained in the requests or responses but typically do not themselves create responses. Filters can also be used to transform the response from the Servlet or JSP before sending it back to client. Filters improve reusability by placing recurring tasks in the filter as a reusable unit. A good way to think of Servlet filters is as a chain of steps that a request and response must go through before reaching a Servlet, JSP, or static resource such as an HTML page in a Web application. F ilte r W e b C o n ta in e r S e r v le t , J S P , H T M L

F ilte r 2 F ilte r 1

Response

Request

F ilte r 3

C li e n t

The filters can be used for caching and compressing content, logging and auditing, image conversions (scaling up or down etc), authenticating incoming requests, XSL transformation of XML content, localization of the request and the response, site hit count etc. The filters are configured through the web.xml file as follows: HitCounterFilter myPkg.HitCounterFilter HitCounterFilter /usersection/* ...

The HitCounterFilter will intercept the requests from the URL pattern /usersection followed by any resource name. Design Pattern: Servlet filters use the slightly modified version of the chain of responsibility design pattern. Unlike the classic (only one object in the chain handle the request) chain of responsibility where filters allow multiple objects (filters) in a chain to handle the request. If you want to modify the request or the response in the chain you can use the decorator pattern (Refer Q11 in How would you go about… section).

Q 23: Explain declarative security for WEB applications? SE

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A 23: Servlet containers implement declarative security. The administration is done through the deployment descriptor

web.xml file. With declarative security the Servlets and JSP pages will be free from any security aware code. You can protect your URLs through web.xml as shown below: web-app> PrivateAndSensitive /private/* executive admin FORM /login.jsp /error.jsp

The user will be prompted for the configured login.jsp when restricted resources are accessed. The container also keeps track of which users have been previously authenticated. Benefits: Very little coding is required and developers can concentrate on the application they are building and system administrators can administer the security settings without or with minimal developer intervention. Let’s look at a sample programmatic security in a Web module like a servlet: CO User user = new User(); Principal principal = request.getUserPrincipal(); if (request.isUserInRole("boss")) user.setRole(user.BOSS_ROLE);

Q 24: Explain the Front Controller design pattern or explain J2EE design patterns? DP A 24: Problem: A J2EE system requires a centralized access point for HTTP request handling to support the integration of system services like security, data validation etc, content retrieval, view management, and dispatching. When the user accesses the view directly without going through a centralized mechanism, two problems may occur: ƒ ƒ ƒ

Each view is required to provide its own system services often resulting in duplicate code. View navigation is left to the views. This may result in shared code for view content and view navigation. Distributed control is more difficult to maintain, since changes will often need to be made in numerous places.

Solution: Generally you write specific servlets for specific request handling. These servlets are responsible for data validation, error handling, invoking business services and finally forwarding the request to a specific JSP view to display the results to the user. J 2 E E F r o n t C o n tr o lle r p a tt e r n d is p a tc h e s

C lie n t

c lie n t re q u e s t

F r o n tC o n tr o lle r

d e le g a te s

A p p lic a tio n F lo w C o n tr o lle r

in v o k e s

< < s e r v l e t> > F r o n tC o n tr o lle r S e r v le t

F r o n tC o n tr o lle r J S P

V ie w

Com m and (e g : S T R U T S A c tio n )

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The Front Controller suggests that we only have one Servlet (instead of having specific Servlet for each specific request) centralising the handling of all the requests and delegating the functions like validation, invoking business services etc to a command or a helper component. For example Struts framework uses the command design pattern to delegate the business services to an action class. Benefits ƒ ƒ ƒ ƒ

Avoid duplicating the control logic like security check, flow control etc. Apply the common logic, which is shared by multiple requests in the Front controller. Separate the system processing logic from the view processing logic. Provides a controlled and centralized access point for your system.

Q 25: Briefly discuss the following patterns Composite view, View helper, Dispatcher view and Service to worker? Or explain J2EE design patterns? DP

A 25: ƒ

Composite View: Creates an aggregate view from atomic sub-views. The Composite View entirely focuses on the View. The View is typically a JSP page, which has the HTML, JSP Tags etc. The JSP display pages mostly have a side bar, header, footer and main content area. These are the sub-views of the view. The subviews can be either static or dynamic. The best practice is to have these sub-views as separate JSP pages and include them in the whole view. This will enable reuse of JSP sub-views and improves maintainability by having to change them at one place only. Com posite View BasicView

1 View

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Com positeView

View Helper: When processing logic is embedded inside the controller or view it causes code duplication in all the pages. This causes maintenance problems, as any change to piece of logic has to be done in all the views. In the view helper pattern the view delegates its processing responsibilities to its helper classes. The helper classes JavaBeans: used to compute and store the presentation data and Custom Tags: used for computation of logic and displaying them iteratively complement each other. Benefits Avoids embedding programming logic in the views and facilitates division of labour between Java developers and Web page designers.

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Service to Worker and Dispatcher View: These two patterns are a combination of Front Controller and View Helper patterns with a dispatcher component. One of the responsibilities of a Front Controller is choosing a view and dispatching the request to an appropriate view. This behaviour can be partitioned into a separate component known as a dispatcher. But these two patterns differ in the way they suggest different division of responsibility among the components. Service to Worker

Dispatcher View

Combines the front controller (Refer Q24 in Enterprise section) and dispatcher, with views and view helpers (refer Q25 in Enterprise section) to handle client requests and dynamically prepares the response.

This pattern is structurally similar to the service to worker but the emphasis is on a different usage pattern. This combines the Front controller and the dispatcher with the view helpers but

ƒ

Controllers delegate the content retrieval to the view helpers, which populates the intermediate model content for the view.

ƒ

Controller does not delegate content retrieval to view helpers because this activity is deferred to view processing.

ƒ

Dispatcher is responsible for the view management and view navigation.

ƒ

Dispatcher is responsible for the view management and view navigation.

Enterprise Java Promotes more up-front work by the front controller and dispatcher for the authentication, authorization, content retrieval, validation, view management and navigation.

77

Relatively has a lightweight front controller and dispatcher with minimum functionality and most of the work is done by the view.

Q 26: Explain Servlet URL mapping? SF Q 26: S e r v le t U R L m a p p in g W ith o u t M a p p in g in w e b .x m l URL U R L eg F ile

h ttp ://< h o s tn a m e :p o rt> /< w e b a p p n a m e > /s e rv le t /< p a th n a m e > /< re s o u rc e n a m e >

h ttp ://lo c a lh o s t:8 0 8 0 /m y A p p s / s e r v le t /m y P a th /M y S e rv le t S E R V E R _ H O M E \W e b A p p s \m y A p p s \W E B -IN F \C la s s e s \m y P a th \M y S e rv le t S e rv e r R o o t D o c u m e n t ro o t

W ith M a p p in g in w e b .x m l d e p lo y m e n t d e s c r ip to r file W e c a n d e fin e th e s e rv le t m a p p in g in th e w e b .x m l d e p lo y m n e t d e s c rip to r file a s s h o w n b e lo w : < w e b -a p p > < s e rv le t> < s e rv le t-n a m e > M y S e r v le t< /s e rv le t-n a m e > < s e rv le t-c la s s > m y P a th .M y S e r v le t< /s e rv le t-c la s s > < /s e rv le t> < s e rv le t-m a p p in g > < s e rv le t-n a m e > M y S e r v le t< /s e rv le t-n a m e > < u rl-p a tte rn > m in e /* .d o < /u rl-p a tte rn > < /s e rv le t-m a p p in g > < w e b -a p p >

U R L a fte r m a p p in g

h ttp ://lo c a lh o s t:8 0 8 0 /m y A p p s /m in e /te s t.d o

N o te : W h ic h m e a n s e v e ry re q u e s t w h ic h h a s a p a tte rn o f h ttp ://lo c a lh o s t:8 0 8 0 /m y A p p s / m in e /* .d o w ill b e h a n d le d b y th e m y P a th .M y S e r v le t c la s s . (* d e n o te s w ild c h a ra c te r fo r a n y a lp h a n u m e ric n a m e ). A ls o p o s s ib le to m a p M y S e rv le t to th e p a tte rn o f /m in e /* , th e * in d ic a te s a n y re s o u rc e n a m e fo llo w e d b y /m in e .

H o w d o w e g e t th e w e b a p p n a m e " m y A p p s " T h e w e b a p p n a m e is d e fin e d in th e a p p lic a tio n .x m l d e p lo y m e n t d e s c rip to r file . T h e < c o n te x t-ro o t > d e n o te s th e w e b a p p n a m e a s s h o w n b e lo w < a p p lic a tio n > ............ < m o d u le id = "W e b M o d u le _ 1 "> < w e b -u ri> m y A p p s W e b .w a r< /w e b -u ri> < c o n te x t-ro o t> m y A p p s < /c o n te x t-ro o t> < /w e b > < /m o d u le > ........ < m o d u le id = "E jb M o d u le _ 1 "> < e jb > m y E J B .ja r< /e jb > < /m o d u le > ..... < /a p p lic a tio n >

Enterprise - JSP

Q 27: What is a JSP? What is it used for? What do you understand by the term JSP translation phase or compilation phase? SF

A 27: JSP (Java ServerPages) is an extension of the Java Servlet technology. JSP is commonly used as the

presentation layer for combining HTML and Java code. While Java Servlet technology is capable of generating HTML with out.println(“….. ”) statements, where out is a PrintWriter. This process of embedding HTML code with escape characters is cumbersome and hard to maintain. The JSP technology solves this by providing a level of abstraction so that the developer can use custom tags and action elements, which can speed up Web development and are easier to maintain.

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As shown in the figure the JSPs have a translation or a compilation process where the JSP engine translates and compiles a JSP file into a JSP Servlet. The translated and compiled JSP Servlet moves to the execution phase (run time) where they can handle requests and send response. Unless explicitly compiled ahead of time, JSP files are compiled the first time they are accessed. On large production sites, or in situations involving complicated JSP files, compilation may cause unacceptable delays to users first accessing the JSP page. The JSPs can be compiled ahead of time (ie precompiled) using application server tools/settings or by writing your own script. J S P m o v in g p a r ts A p p lic a tio n S e r v e r W e b C o n ta in e r

S e rv le ts

JSP Engine

Web Server

CLIENT (Browser)

Servlet Engine

HTTP re q u e s t

JSPs

J S P S e rv le ts

HTTP re s p o n s e s ta tic d o c s (H T M L ,C S S e tc )

Q 28: Explain the life cycle methods of a JSP? SF A 28: ƒ ƒ ƒ ƒ ƒ

Pre-translated: Before the JSP file has been translated and compiled into the Servlet. Translated: The JSP file has been translated and compiled as a Servlet. Initialized: Prior to handling the requests in the service method the container calls the jspInit() to initialize the Servlet. Called only once per Servlet instance. Servicing: Services the client requests. Container calls this method for each request. Out of service: The Servlet instance is out of service. The container calls the jspDestroy() method.

Q 29: What are the main elements of JSP? What are scriplets? What are expressions? SF A 29: There are two types of data in a JSP page. ƒ ƒ

Static part (ie HTML, CSS etc), which gets copied directly to the response by the JSP Engine. Dynamic part, which contains anything that can be translated and compiled by the JSP Engine.

There are three types of dynamic elements. (TIP: remember SAD as an abbreviation for Scripting, Action and Directive elements). Scripting Elements: A JSP element that provides embedded Java statements. There are three types of scripting elements. ƒ

Declaration Element: is the embedded Java declaration statement, which gets inserted at the Servlet class level. Important: declaring variables via this element is not thread-safe, because this variable ends up in the generated Servlet as an instance variable, not within the body of the _jspservice() method. Ensure their access is either read-only or synchronized.

ƒ

Expression Element: is the embedded Java expression, which gets evaluated by the service method.

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Scriptlet Elements: are the embedded Java statements, which get executed as part of the service method. (Note: Not recommended to use Scriptlet elements because they don’t provide reusability and maintainability. Use custom tags (like JSTL, JSF tags, etc) or beans instead).

Action Elements: A JSP element that provides information for execution phase.

Directive Elements: A JSP element that provides global information for the translation phase.

Q 30: What are the different scope values or what are the different scope values for ? SF A 30: Scope

Object

Comment

Page Request Session Application

PageContext Request Session Application

Available to the handling JSP page only. Available to the handling JSP page or Servlet and forwarded JSP page or Servlet. Available to any JSP Page or Servlet within the same session. Available to all the JSP pages and Servlets within the same Web Application.

Q 31: What are the differences between static and a dynamic include? SF DC A 31: Static include

Dynamic include

During the translation or compilation phase all the included JSP pages are compiled into a single Servlet.

The dynamically included JSP is compiled into a separate Servlet. It is a separate resource, which gets to process the request, and the content generated by this resource is included in the JSP response. Has run time performance overhead.

No run time performance overhead.

Which one to use: Use “static includes” when a JSP page does not change very often. For the pages, which change frequently, use dynamic includes. JVM has a 64kb limit on the size of the method and the entire JSP page is rendered as a single method. If a JSP page is greater than 64kb, this probably indicates poor implementation. When this method reaches its limit of 64kb it throws an error. This error can be overcome by splitting the JSP files and including them dynamically (i.e. using ) because the dynamic includes generate separate JSP Servlet for each included file. Note: The “dynamic include” (jsp:include) has a flush attribute. This attribute indicates whether the buffer should be flushed before including the new content. In JSP 1.1 you will get an error if you omit this attribute. In JSP 1.2 you can omit this attribute because the flush attribute defaults to false.

Q 32: What are implicit objects and list them? SF A 32: Implicit objects are the objects that are available for the use in JSP documents without being declared first. These objects are parsed by the JSP engine and inserted into the generated Servlet. The implicit objects are: Implicit object

Scope

comment

request response pageContext session application out config page exception

Request Page Page Session Application Page Page Page Page

request response page environment session same as ServletContext writing to the outputstream same as ServletConfig this page’s Servlet exception created on this page.

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Note: Care should be taken not to name your objects the same name as the implicit objects. If you have your own object with the same name, then the implicit objects take precedence over your own object.

Q 33: Explain hidden and output comments? SF A 33: An output comment is a comment that is sent to the client where it is viewable in the browser’s source. CO

A hidden comment documents a JSP page but does not get sent to the client. The JSP engine ignores a hidden comment, and does not process any code within hidden comment tags.

Q 34: Is JSP variable declaration thread safe? CI A 34: No. The declaration of variables in JSP is not thread-safe, because the declared variables end up in the generated Servlet as an instance variable, not within the body of the _jspservice() method. The following declaration is not thread safe: because these are declarations, and will only be evaluated once when the page is loaded

The following declaration is thread safe: because the variables declared inside the scriplets have the local scope and not shared.

Q 35: Explain JSP URL mapping? What is URL hiding or protecting the JSP page? SF SE A 35: As shown in the figure, the JSP resources usually reside directly or under subdirectories (e.g. myPath) of the

document root, which are directly accessible to the user through the URL. If you want to protect your Web resources then hiding the JSP files behind the WEB-INF directory can protect the JSP files, css (cascading style sheets) files, Java Script files, pdf files, image files, html files etc from direct access. The request should be made to a servlet who is responsible for authenticating and authorising the user before returning the protected JSP page or its resources. JSP URL M apping URL

URL eg

File

http:////

http://localhost:8080/m yApps/m yPath/m yPage.jsp

SERVER_HO M E\W ebApps\m yApps\m yPath\m yPage.jsp Server Root

Unhidden URL

Docum ent root

m yPage.jsp is dircetly accessible through URL MyApps

Hidden URL m yPage.jsp is hidden or protected. cannot be directly accessed through URL MyApps

m yPath

m yPage.jsp

W EB-INF lib classes

W EB-INF m yPath lib classes

m yPage.jsp

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Q 36: What are custom tags? Explain how to build custom tags? SF A 36: Custom JSP tag is a tag you define. You define how a tag, its attributes and its body are interpreted, and then group your tags into collections called tag libraries that can be used in any number of JSP files. So basically it is a reusable and extensible JSP only solution. The pre-built tags also can speed up Web development. CO STEP: 1 Construct the Tag handler class that defines the behaviour. T a g E v a lu a tio n P ro c e s s S a m p le c o d e u s in g o n ly d o S a ta rtT a g ()

s ta rt

p a c k a g e m yT a g P k g ; C a ll s e tX X X () m e th o d s o n th e T a g

d o S ta rtT a g () e v a lu a te b o d y o f th e T a g

p u b lic c la s s M yT a g e x te n d s T a g S u p p o rt { in t a ttr = n u ll; p u b lic in t s e tA ttr(in t a ttr){th is .a ttr = a ttr} p u b lic in t g e tA ttr(){re tu rn a ttr;}

lo o p

p u b lic in t d o S ta rtT a g () th ro w s J s p E x c e p tio n { ....... re tu rn S K IP _ B O D Y ; }

d o A fte rB o d y () d o E n d T a g ()

p u b lic v o id re le a s e (){.....} re le a s e ()

}

STEP: 2 The Tag library descriptor file (*.tld) maps the XML element names to the tag implementations. The code sample MyTagDesc.tld is shown below: tag1 myTagPkg.MyTag empty attr false false

STEP: 3 The web.xml deployment descriptor maps the URI to the location of the *.tld (Tag Library Descriptor) file. The code sample web.xml file is shown below: /WEB-INF/MyTagURI /WEB-INF/tags/MyTagDesc.tld

STEP: 4 The JSP file declares and then uses the tag library as shown below: < myTag:tag1 attr=”abc”> or < myTag:tag1 attr=”abc” />

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Q 37: What is a TagExtraInfo class? SF A 37: A TagExtraInfo class provides extra information about tag attributes to the JSP container at translation time. ƒ

Returns information about the scripting variables that the tag makes available to the rest of the JSP page to use. The method used is: VariableInfo[] getVariableInfo(TagData td)

Example

Without the use of TagExtraInfo, if you want to manipulate the attribute myArray in the above code in a scriptlet it will not be possible. This is because it does not place the myArray object on the page. You can still use pageContext.getAttribute() but that may not be a cleaner approach because it relies on the page designer to correctly cast to object type. The TagExtraInfo can be used to make items stored in the pageContext via setAttribute() method available to the scriptlet as shown below. = (greater than or equal), No Mucho Sample.dtd Sample.xsd

XSL

XSL stands for eXtensible Stylesheet Language. The XSL consists of 3 parts:

To convert the Sample.xml file to a XHTML file let us apply the following Sample.xsl through XALAN parser.

•

XSLT: Language for transforming XML documents from one to another.

Sample.xsl

•

XPath: Language for defining the parts of an XML document.

•

XSL-FO: Language for formatting XML documents. For example to convert an XML document to a PDF document etc.

XSL can be thought of as a set of languages that can : • • • •

Define parts of an XML Transform an XML document to XHTML (eXtensible Hyper Text Markup Language) document. Convert an XML document to a PDF document. Filter and sort XML data.

You get the following output XHTML file:

XSLT processor example: Xalan (From Apache) PDF Processor example: Processor from Apache)

FOP (Formatting Objects

Sample.xhtml English

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Now to convert the Sample.xml into a PDF file apply the following FO (Formatting Objects) file Through the FOP processor. Sample.fo which gives a basic Sample.pdf which has the following line Not Much XPath

JAXP

Xml Path Language, a language for addressing parts of an XML document, designed to be used by both XSLT and XPointer. We can write both the patterns (context-free) and expressions using the XPATH Syntax. XPATH is also used in XQuery. Stands for Java API for XML Processing. This provides a common interface for creating and using SAX, DOM, and XSLT APIs in Java regardless of which vendor’s implementation is actually being used (just like the JDBC, JNDI interfaces). JAXP has the following packages:

As per Sample.xsl ……… DOM example using JAXP: DocumentBuilderFactory dbf = DocumentBuilderFactory.newInstance(); DocumentBuilder db = dbf.newDocumentBuilder(); Document doc = db.parse(new File("xml/Test.xml")); NodeList nl = doc.getElementsByTagName("to"); Node n = nl.item(0); System.out.println(n.getFirstChild().getNodeValue()); SAX example using JAXP: SAXParserFactory spf = SAXParserFactory.newInstance(); SAXParser sp = spf.newSAXParser(); SAXExample se = new SAXExample(); sp.parse(new File("xml/Sample.xml"),se); where SAXExample.Java code snippet public class SAXExample extends DefaultHandler { public void startElement( String uri, String localName, String qName, Attributes attr) throws SAXException { System.out.println("--->" + qName); } ... }

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The DefaultHandler implements ContentHandler, DTDHandler, EntityResolver, ErrorHandler

JAXP implements

SAXParser Factory

XML Sample. xml

SAXReader SAXExample

SAXParser

Content Handler

StreamSource xml = new StreamSource(new File("/xml/Sample.xml")); StreamSource xsl = new StreamSource( new File("xml/Sample.xsl")); StreamResult result = new StreamResult(new File("xml/Sample.xhtml"));

Error Handler DTD Handler Entity Resolver

TransformerFactory tf = TransformerFactory.newInstance(); Transformer t = tf.newTransformer(xsl); t.transform(xml, result);

DocumentBuilder Factory

XML Sample. xml

This gives you Sample.xhtml Document (DOM)

Document Builder

XSLT example using JAXP:

note to

English

from

Transformer Factory Source sample. xml

Transformer

Result sample. xhtml

Transformation instructions sample.xsl

• • • •

JAXB

javax.xml.parsers Æ common interface for different vendors of SAX, DOM parsers). org.xml.sax Æ Defines basic SAX API. org.w3c.dom Æ Defines Document Object Model and its componenets. javax.xml.transform Æ Defines the XSLT API which allows you to transform XML into other forms like PDF, XHTML etc.

Required JAR files are jaxp.jar, dom.jar, xalan.jar, xercesImpl.jar. Stands for Java API for XML Binding. This standard defines a mechanism for writing out Java objects as XML (marshalling) and for creating Java objects from XMLstructures (unmarshalling). (You compile a class description to create the Java classes, and use those classes in your application.)

Lets look at some code: For binding: xjc.sh –p com.binding sample.xsd –d work -p identifies the package for the generated Java files (ie *.Java) -d option identifies the target. Unmarshalling the XML document: JAXBContext jc = JAXBContext.newInstance( “com.binding”); Unmarshaller um = jc.createUnmarshaller(); Object o = um.unMarshall( new File(“/xml/”));

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Note n = (Note) n; System.out.println(n.getFrom().getContent().get(0)); System.out.println(n.getTo().getContent().get(0));

JAXB XML schema Sample.xsd

Now to modify the in memory object content:

xjc binding compiler

n. getFrom().getContent().set(0, “newValue”); Marshalling the change back to different XML file: Marshaller m = jc.createMarshaller(); FileOutputStream fos = new FileOutputStream( new File(“/xml/SampleNew.xml”));

Java files

m.marshall(n, fos);

javac

(*.java interfaces & implementations)

us e

Java class files *.class

Application XML Sample. xml

unmarshall JAXB API

marshall

Java content Objects

note to

from

Enterprise – SQL, Tuning and O/R mapping

Q 96: Explain inner and outer joins? SF A 96: Joins allow database users to combine data from one table with data from one or more other tables (or views, or synonyms). Tables are joined two at a time making a new table containing all possible combinations of rows from the original two tables. Lets take an example (syntax vary among RDBMS):

Joins and Set operations in Relational Databases

Inner join

Left outer join

Right outer join

Full outer join

Enterprise Java

120 Employees table Id 1001 1002 1003 1004 Executives table Id 1001 1002 1005

firstname John Peter Paul Sam

Surname Darcy Smith Gregor Darcy

state NSW NSW NSW VIC

firstname John Peter John

Surname Darcy Smith Gregor

state NSW NSW WA

Inner joins: Chooses the join criteria using any column names that happen to match between the two tables. The example below displays only the employees who are executives as well. SELECT emp.firstname, exec.surname FROM employees emp, executives exec WHERE emp.id = exec.id; The output is: Firstname John Peter

surname Darcy Smith

Left Outer joins: A problem with the inner join is that only rows that match between tables are returned. The example below will show all the employees and fill the null data for the executives. SELECT emp.firstname, exec.surname FROM employees emp left join executives exec ON emp.id = exec.id;

On oracle SELECT emp.firstname, exec.surname FROM employees emp, executives exec where emp.id = exec.id(+);

The output is: Firstname John Peter Paul Sam

surname Darcy Smith

Right Outer join: A problem with the inner join is that only rows that match between tables are returned. The example below will show all the executives and fill the null data for the employees. SELECT emp.firstname, exec.surname FROM employees emp right join executives exec ON emp.id = exec.id; On oracle SELECT emp.firstname, exec.surname FROM employees emp, executives exec WHERE emp.id(+) = exec.id; The output is: Firstname John Peter

surname Darcy Smith Gregor

Full outer join: To cause SQL to create both sides of the join SELECT emp.firstname, exec.surname FROM employees emp full join executives exec ON emp.id = exec.id;

On oracle SELECT emp.firstname, exec.surname FROM employees emp, executives exec WHERE emp.id = exec.id (+)

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UNION SELECT emp.firstname, exec.surname FROM employees emp, executives exec WHERE emp.id(+) = exec.id Note: Oracle9i introduced the ANSI compliant join syntax. This new join syntax uses the new keywords inner join, left outer join, right outer join, and full outer join, instead of the (+) operator.

The output is: Firstname John Paul Peter Sam

surname Darcy Smith Gregor

Self join: A self-join is a join of a table to itself. If you want to find out all the employees who live in the same city as employees whose first name starts with “Peter”, then one way is to use a sub-query as shown below: SELECT emp.firstname, emp.surname FROM employees emp WHERE city IN (SELECT city FROM employees where firstname like ‘Peter’)

The sub-queries can degrade performance. So alternatively we can use a self-join to achieve the same results. On oracle SELECT emp.firstname, emp.surname FROM employees emp, employees emp2 WHERE emp.state = emp2.state AND emp2.firstname LIKE 'Peter'

The output is: Firstname John Peter Paul

Surname Darcy Smith Gregor

Q 97: Explain a sub-query? How does a sub-query impact on performance? SF PI A 97: It is possible to embed a SQL statement within another. When this is done on the WHERE or the HAVING statements, we have a subquery construct. What is subquery useful for? It is used to join tables and there are cases where the only way to correlate two tables is through a subquery. SELECT emp.firstname, emp.surname FROM employees emp WHERE emp.id NOT IN (SELECT id FROM executives);

There are performance problems with sub-queries, which may return NULL values. The above sub-query can be re-written as shown below by invoking a correlated sub-query: SELECT emp.firstname, emp.surname FROM employees emp WHERE emp.id NOT EXISTS (SELECT id FROM executives);

The above query can be re-written as an outer join for a faster performance as shown below: SELECT emp.firstname, exec.surname FROM employees emp left join executives exec on emp.id = exec.id AND exec.id IS NULL;

The above execution plan will be faster by eliminating the sub-query.

Q 98: What is normalization? When to denormalize? DC PI A 98: Normalization is a design technique that is widely used as a guide in designing relational databases. Normalization is essentially a two step process that puts data into tabular form by removing repeating groups and then removes duplicated data from the relational tables (Additional reading recommended). Redundant data wastes disk space and creates maintenance problems. If data that exists in more than one place must be changed, the data must be changed in exactly the same way in all locations which is time consuming and prone to errors. A change to a customer address is much easier to do if that data is stored only in the Customers table and nowhere else in the database.

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Inconsistent dependency is a database design that makes certain assumptions about the location of data. For example, while it is intuitive for a user to look in the Customers table for the address of a particular customer, it may not make sense to look there for the salary of the employee who calls on that customer. The employee's salary is related to, or dependent on, the employee and thus should be moved to the Employees table. Inconsistent dependencies can make data difficult to access because the path to find the data may not be logical, or may be missing or broken. First normal form

Second Normal Form

Third Normal Form

A database is said to be in First Normal Form when all entities have a unique identifier or key, and when every column in every table contains only a single value and doesn't contain a repeating group or composite field.

A database is in Second Normal Form when it is in First Normal Form plus every non-primary key column in the table must depend on the entire primary key, not just part of it, assuming that the primary key is made up of composite columns.

A database is in Third Normal Form when it is in Second Normal Form and each column that isn't part of the primary key doesn't depend on another column that isn't part of the primary key.

When to denormalize? Normalize for accuracy and denormalize for performance. Typically, transactional databases are highly normalized. This means that redundant data is eliminated and replaced with keys in a one-to-many relationship. Data that is highly normalized is constrained by the primary key/foreign key relationship, and thus has a high degree of data integrity. Denormalized data, on the other hand, creates redundancies; this means that it's possible for denormalized data to lose track of some of the relationships between atomic data items. However, since all the data for a query is (usually) stored in a single row in the table, it is much faster to retrieve.

Q 99: How do you implement one-to-one, one-to-many and many-to-many relationships while designing tables? SF A 99: One-to-One relationship can be implemented as a single table and rarely as two tables with primary and foreign key relationships. One-to-Many relationships are implemented by splitting the data into two tables with primary key and foreign key relationships. Many-to-Many relationships are implemented using join table with the keys from both the tables forming the composite primary key of the junction table.

Q 100: How can you performance tune your database? PI A 100: ƒ ƒ ƒ ƒ ƒ

Denormalize your tables where appropriate. Proper use of index columns: An index based on numeric fields is more efficient than an index based on character columns. Reduce the number of columns that make up a composite key. Proper partitioning of tablespaces and create a special tablespace for special data types like CLOB, BLOB etc. Data access performance can be tuned by using stored procedures to crunch data in the database server to reduce the network overhead and also caching data within your application to reduce the number of accesses.

Q 101: How will you map objects to a relational database? How will you map class inheritance to relational data model? DC

A 101: Due to impedance mismatch between object and relational technology you need to understand the process of mapping classes (objects) and their relationships to tables and relationships between them in a database. Classes represent both behaviour and data whereas relational database tables just implement data. Database schemas have keys (primary keys to uniquely identify rows and foreign keys to maintain relationships between rows) whereas object schema does not have keys and instead use references to implement relationships to other objects. Let us look at some basic points on mapping:

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Object to Relational (O/R) mapping of class inheritance structure Map class hierarchy to a single database table

Class diagram

Address -streetname

Employee

Employee

-EmployeeId (PK) -EmployeeType -Name -Salary -Rate -Period

-EmployeeId (PK) -IsPermanent -IsContractor -IsSubContractor -Name -Salary -Rate -Period

Refactored

Employee *1

Permanent

Map each class to its own table

Employee

-name -address +getName()

-EmployeeId (PK) -Name

Contractor

-salary

-rate

+getSalary()

+getRate()

Permanent

Contractor

-EmployeeId (PK) -Salary

-EmployeeId (PK) -Contract

SubContractor

SubContractor

-period

-EmployeeId (PK) -Period

+calculateTotal()

Map each concrete class to its own table Permanent

Contractor

-EmployeeId (PK) -Name -Salary

-EmployeeId (PK) -Name -Contract

SubContractor -EmployeeId (PK) -Name -Period

ƒ ƒ ƒ

ƒ

Classes map to tables in a way but not always directly. An attribute of a class can be mapped to zero or more columns in a database. Not all attributes are persistent. Some attributes of an object are objects itself. For example an Employee object has an Address object as an attribute. This is basically an association relationship between two objects (i.e. Employee and Address). This is a recursive relationship where at some point the attribute will be mapped to zero or more columns. In this example attributes of the Address class will be mapped zero or more columns. In its simple form an attribute maps to a single column whereas each has same type (ie attribute is a string and column is a char, or both are dates etc). When you implement mapping with different types (attribute is a currency and column is a float) then you will need to be able to convert them back and forth.

How do you map inheritance class structure to relational data model? Relational databases do not support inheritance. Class inheritance can be mapped to relational tables as follows: Map class hierarchy to single database table: The whole class hierarchy can be stored in a single table by adding an additional column named “EmployeeType”. The column “EmployeeType” will hold the values “Permanent”, “Contract” and “SubContract”. New employee types can be added as required. Although this approach is straightforward it tends to break when you have combinations like an employee is of type both “Contractor” and “SubContractor”. So when you have combinations, you can use refactored table by replacing type code column “EmployeeType” with boolean values such as isPermanent, isContractor and isSubContractor. Map each class to its own table: You create one table per class. The data for a permanent employee is stored in two tables (Employee and Permanent), therefore to retrieve this data you need to join these two tables. To support additional employee type say a Contractor, add a new table.

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Map each concrete class to its own table: You create one table per concrete class. There are tables corresponding to each class like Permanent, Contractor and SubContractor. So join is not required. To support additional employee type, add a new table. So which approach to use? Easiest approach is to have one table per hierarchy and easy to refactor. If you need a “pure design approach” then use one table per class approach. Try to stay away from one table per concrete class approach because it makes refactoring difficult by copying data back and forth between tables. No approach is ideal for all situations. Another option for mapping inheritance into relational database is to take a generic meta-data driven approach. This approach supports all forms of mapping. In this approach, value of a single attribute will be stored as a row in a table called “Value”. So, to store 5 attributes you need 5 rows in “Value” table. You will have a table called “Class” where class names are stored, a table called “Inheritance” where subclass and superclass information is stored, a table called “Attributes” where class attributes are stored and an “AttributeType” lookup table.

Q 102: What is a view? Why will you use a view? What is an aggregate function? Etc. SF PI A 102: Question

Explanation

What is view? Why use a view?

View is a precompiled SQL query, which is used to select data from one or more tables. A view is like a table but it doesn’t physically take any space (ie not materialised). Views are used for ƒ ƒ ƒ

Providing inherent security by exposing only the data that is needed to be shown to the end user. Enabling re-use of SQL statements. Allows changes to the underlying tables to be hidden from clients, aiding maintenance of the database schema (i.e. encapsulation).

Views with multiple joins and filters can dramatically degrade performance because views contain no data and any retrieval needs to be processed. The solution for this is to use materialised views or create de-normalised tables to store data. This technique is quite handy in overnight batch processes where a large chunk of data needs to be processed. Normalised data can be read and inserted into some temporary denormalised table and processed with efficiency. Explain aggregate SQL functions?

SQL provides aggregate functions to assist with the summarisation of large volumes of data. We’ll look at functions that allow us to add and average data, count records meeting specific criteria and find the largest and smallest values in a table. ORDERID 1001 1002 1003

FIRSTNAME John Peter Sam

SURNAME Darcy Smith Gregory

QTY 25 25 25

UNITPRICE 10.5 10.5 10.5

SELECT SUM(QTY) AS Total FROM Orders; The output is: Total = 75 SELECT AVG(UnitPrice * QTY) As AveragePrice FROM Orders; The output is: AveragePrice = 262.50 If we inserted another row to the above table: ORDERID 1004

FIRSTNAME John

SURNAME Darcy

QTY 20

UNITPRICE 10.50

SELECT FIRSTNAME,SUM(QTY) FROM orders GROUP BY FIRSTNAME HAVING SUM(QTY)>25; The output is: John 45 Explain INSERT, UPDATE, and DELETE statements?

INSERT statements can be carried out several ways: INSERT INTO ORDERS values (1004, 'John', 'Darcy', 20, 10.50);

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The above statement is fine but the one below is recommended since it is less ambiguous and less prone to errors. INSERT INTO ORDERS (orderid, firstname, surname, qty, unitprice) values (1005, 'John', 'Darcy', 20, 10.50); We can also use INSERT with the SELECT statements as shown below INSERT into NEW_ORDERS (orderid, firstname, surname, qty, unitprice) SELECT orderid, firstname, surname, qty, unitprice FROM orders WHERE orderid = 1004; UPDATE statement allows you to update a single or multiple statements. UPDATE ORDERS set firstname='Peter', surname='Piper' WHERE orderid=1004; Also can have more complex updates like UPDATE supplier SET supplier_name = ( SELECT customer.name FROM customers WHERE customers.customer_id = supplier.supplier_id) WHERE EXISTS (SELECT customer.name FROM customers WHERE customers.customer_id = supplier.supplier_id); DELETE statements allow you to remove records from the database. DELETE FROM ORDERS WHERE orderid=1004; We can clear the entire table using TRUNCATE TABLE employees; When running UPDATE/DELETE care should be taken to include WHERE clause otherwise you can inadvertently modify or delete records which you do not intend to UPDATE/DELETE. How can you compare a part of the name rather than the entire name?

You can use wild card characters like: • •

* ( % in oracle) Æ Match any number of characters. ? ( _ in oracle) Æ Match a single character.

To find all the employees who has “au”: SELECT * FROM employees emp WHERE emp.firstname LIKE ‘%au%’; How do you get distinct entries from a table?

The SELECT statement in conjunction with DISTINCT lets you select a set of distinct values from a table in a database. SELECT DISTINCT empname FROM emptable

How can you find the total number of records in a table?

Use the COUNT key word: SELECT COUNT(*) FROM emp WHERE age>25

What's the difference between a primary key and a unique key?

Both primary key and unique key enforce uniqueness of the column on which they are defined. But by default primary key creates a clustered index on the column, whereas unique creates a non-clustered index by default. Another major difference is that, primary key doesn't allow NULLs, but unique key allows one NULL only.

What are constraints? Explain different types of constraints.

Constraints enable the RDBMS enforce the integrity of the database automatically, without needing you to create triggers, rule or defaults. Types of constraints: NOT NULL, CHECK, UNIQUE, PRIMARY KEY, FOREIGN KEY

What is an index? What are the types of indexes? How many clustered indexes can be created on a table? What are the

The books you read have indexes, which help you to go to a specific key word faster. The database indexes are similar. Indexes are of two types. Clustered indexes and non-clustered indexes. When you

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create a clustered index on a table, all the rows in the table are stored in the order of the clustered index key. So, there can be only one clustered index per table. Non-clustered indexes have their own storage separate from the table data storage. The row located could be the RowID or the clustered index key, depending up on the absence or presence of clustered index on the table. If you create an index on each column of a table, it improves the query performance, as the query optimizer can choose from all the existing indexes to come up with an efficient execution plan. At the same time, data modification operations (such as INSERT, UPDATE, and DELETE) will become slow, as every time data changes in the table, all the indexes need to be updated. Another disadvantage is that, indexes need disk space, the more indexes you have, more disk space is used.

Enterprise - RUP & UML

Q 103: What is RUP? SD A 103: Rational Unified Process (RUP) is a general framework that can be used to describe a development process.

The software development cycle has got 4 phases in the following order Inception, Elaboration, Construction, and Transition. R a t io n a l U n if ie d P r o c e s s PHASES

Disciplines (vertical axis)

B u s in e s s M o d e llin g

In c e p tio n

e la b o r a tio n

c o n s tr u c tio n

In c e p tio n # 1

E la b # 1

C on #1

tr a n s itio n

R e q u ir e m e n ts A n a ly s is & D e s ig n Im p le m e n ta tio n T e s t D e p lo y m e n t C o n f ig & C h a n g e m g m t P r o je c t m g m t E n v ir o n m e n t

E la b # 2

C on #2

C on #3

T r a n s itio n # 1

IT E R A T IO N S

P H A S E S / I T E R A T IO N S ( H o r iz o n t a l a x is )

The core of the phases is state-based, and the state is determined by what fundamental questions you are trying to answer: ƒ ƒ ƒ ƒ

Inception - do you and the customer have a shared understanding of the system? Elaboration - do you have baseline architecture to be able to build the system? Construction - are you developing a product? Transition - are you trying to get the customer to take ownership of the system?

RUP is based on a few important philosophies and principles: ƒ ƒ ƒ

A software project team should plan ahead. It should know where it is going. It should capture project knowledge in a storable and extensible form.

The best practices of RUP involve the following major 5 properties: Best practice property

Description

Use case driven

Interaction between the users and the system.

Architecture centric Iterative

Based on architecture with clear relationships between architectural components. The problem and the solution are divided into more manageable smaller pieces, where each

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iteration will be addressing one of those pieces. Incremental

Each iteration builds incrementally on the foundation built in the previous iteration.

Controlled

Control with respect to process means you always know what to do next; control with respect to management means that all deliverables, artifacts, and code are under configuration management.

Q 104: Explain the 4 phases of RUP? SD A 104:

RUP Phases

Construction INCEPTION

ELABORATION

TRANSITION Iter 1 Iter 2 Iter 3 Iter 4 Iter 5 Iter 6

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Inception: During the inception phase, you work out the business case for the project. You also will be making a rough cost estimate and return on investment. You should also outline the scope and size of the project. The fundamental question you ask at the end of this phase: do you and the customer have a shared understanding of the system?

ƒ

Elaboration: At this stage you have the go ahead of the project however only have vague requirements. So at this stage you need to get a better understanding of the problem. Some of the steps involved are: ƒ ƒ ƒ ƒ ƒ

What is it you are actually going to build? How are you going to build it? What technology are you going to use? Analysing and dealing with requirement risks, technological risks, skill risks, political risks etc. Develop a domain model, use case model and a design model. The UML techniques can be used for the model diagrams (e.g. class diagrams, sequence diagrams etc).

An important result of the elaboration phase is that you have a baseline architecture. This architecture consists of: ƒ ƒ ƒ

A list of use cases depicting the requirements. The domain model, which captures your understanding of the domain with the help of UML class diagrams. Selection of key implementation technology and how they fit together. For example: Java/J2EE with JSP, Struts, EJB, XML, etc.

The fundamental question you ask at the end of this phase: do you have a baseline architecture to be able to build the system? ƒ

Construction: In this phase you will be building the system in a series of iterations. Each iteration is a mini project. You will be performing analysis, design, unit testing, coding, system testing, and integration testing for the use cases assigned to each iteration. The iterations within the construction phase are incremental and iterative. Each iteration builds on the use cases developed in the previous iterations. The each iteration will involve code rewrite, refactoring, use of design patterns etc. The basic documentation required during the construction phase is: ƒ ƒ

A class diagram and a sequence diagram. Some text to pull the diagrams together.

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If a class has complex life cycle behaviour then a state diagram is required. If a class has a complex computation then an activity diagram is required.

The fundamental question you ask at the end of this phase: do you have a developed product? Transition: During this phase you will be delivering the finished code regularly. During this phase there is no coding to add functionality unless it is small and essential. There will be bug fixes, code optimisation etc during this phase. An example of a transition phase is that the time between the beta release and the final release of a product.

ƒ

The fundamental question you ask at the end of this phase: are you trying to get the customer to take ownership of the developed product or system?

Q 105: What are the characteristics of RUP? Where can you use RUP? SD A 105: 1. RUP is based on a few important philosophies and principles like planning ahead, knowing where the process is heading and capturing the project in storable and extensible manner. 2. It is largely based on OO analysis and design, and use case driven etc. 3. Iterative and incremental development as oppose to waterfall approach, which hides problems. 4. Architecture centric approach. RUP is more suited for larger teams of 50-100 people. RUP can also be used as an agile (i.e. lightweight) process for smaller teams of 20-30 people, or as a heavy weight process for larger teams of 50-100 people. Extreme Programming (XP) can be considered as a subset of RUP. At the time of writing, the agile (i.e lightweight) software development process is gaining popularity and momentum across organizations. Several methodologies fit under this agile development methodology banner. All these methodologies share many characteristics like iterative and incremental development, test driven development, stand up meetings to improve communication, automatic testing, build and continuous integration of code etc. Refer Q136 in Enterprise Java section.

Q 106: Why is UML important? SD DC A 106: The more complicated the underlying system, the more critical the communication among everyone involved in developing and deploying the software. UML is a software blueprint language for analysts, designers and developers. UML provides a common vocabulary for the business analysts, architects, developers etc. UML is applicable to the Object Oriented problem solving. UML begins with a model; A model is an abstraction of the underlying problem. The domain is the actual world from which the problem comes. The model consists of objects. The objects interact with each other by sending and receiving messages. The objects are characterised by attributes and operations (behaviours). The values of an object’s attributes determine its state. The classes are the blueprints (or like templates) for objects. A class wraps attributes and methods into a single distinct entity. The objects are the instances of classes.

Q 107: What are the different types of UML diagrams? SD DC A 107: Use case diagrams: Depicts the typical interaction between external users (actors) and the system. The emphasis is on what a system does rather than how it does it. A use case is a summary of scenarios for a single task or goal. An actor is responsible for initiating a task. The connection between actor and use case is a communication association. Capturing use cases is one of the primary tasks of the elaboration phase of RUP. In its simplest usage, you capture a use case by talking to your users and discussing the various things they might want to do with the system. When to use ‘use case’ diagrams? ƒ ƒ ƒ

Determining user requirements. New use cases often generate new requirements. Communicating with clients. The simplicity of the diagram makes use case diagrams a good way for designers and developers to communicate with clients. Generating test cases. Each scenario for the use case may suggest a suite of test cases.

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U s e c a s e d ia g r a m

a s s o c ia tio n *

*

< < in c lu d e > >

E n r o ll in U n iv e r s i ty

R e g is tr a r

E n r o ll in s e m in a r

a s s o c ia tio n

< < e x te n d > > S tu d e n t

in h e r ita n c e

p e r fo r m s e c u r ity ch e ck

E n r o ll fa m ily m e m b e rs

in h e r ita n c e N o te : < < e x t e n d > > r e la tio n s h ip is c o n d itio n a l. Y o u d o n o t k n o w if o r w h e n e x t e n d in g u s e c a s e w ill b e in v o k e d . < < in c lu d e > > r e la tio n s h ip is s im ila r t o a p r o c e d u r e c a ll. i n h e r it a n c e : e x t e n d s t h e b e h a v io r o f th e p a re n t u s e c a s e o r a c to r.

a s s o c ia t io n

In te r n a tio n a l S tu d e n t

Class diagrams: Class diagram technique is vital within Object Oriented methods. Class diagrams describe the types of objects in the system and the various static relationships among them. Class diagrams also show the attributes and the methods. Class diagrams have the following possible relationships: ƒ

Association: A relationship between instances of 2 classes.

ƒ

Aggregation: An association in which one class belongs to a collection (does not always have to be a collection. You can also have cardinality of “1”). This is a part of a whole relationship where the part can exist without the whole. For example: A line item is whole and the products are the parts. If a line item is deleted then the products need not be deleted.

ƒ

Composition: An association in which one class belongs to a collection (does not always have to be a collection. You can also have cardinality of “1”). This is a part of a whole relationship where the part cannot exist without the whole. If the whole is deleted then the parts are deleted. For example: An Order is a whole and the line items are the parts. If an order is deleted then all the line items should be deleted as well (ie cascade deletes).

ƒ

Generalization: An inheritance link indicating that one class is a superclass of the other. The Generalization expresses the “is a” relationship whereas the association, aggregation and composition express the “has a” relationship.

ƒ

Dependency: A dependency is a weak relationship where one class requires another class. The dependency expresses the “uses” relationship. For example: A domain model class uses a utility class like Formatter etc. C la s s D ia g ra m a s s o c ia tio n

c o m p o s itio n

C u s to m e r -o rd e r : O rd e r

d ep endency

O rd e rD e ta il

O rd e r 1

*

-lin e Ite m s : L is t

1

1 ..*

+ o p e ra tio n 1 ()

g e n e ra liz a tio n S ta n d a rd O rd e r

C u s to m O rd e r -s p e c ia lD e ta ils

« u tility » F o rm a tte r + fo rm a tD e c im a l() + fo rm a tC u rre n c y()

-L in e Id -p ro d u c t : P ro d u c t -c o m m e n ts

1 * P ro d u c t

-p ro d u c tC o d e -p ro d u c tD e s c -u n itP ric e

When to use class diagrams? ƒ

a g g re g a tio n

Class diagrams are the backbone of Object Oriented methods. So they are used frequently.

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Class diagrams can have a conceptual perspective and an implementation perspective. During the analysis draw the conceptual model and during implementation draw the implementation model.

Package diagrams: To simplify complex class diagrams you can group classes into packages. P a c k a g e D ia g r a m

A c c o u n tin g

O r d e r in g

d ep en d en cy d ep en d en cy C u s to m e r

When to use package diagrams? ƒ

Package diagrams are vital for large projects.

Object diagrams: Object diagrams show instances instead of classes. They are useful for explaining some complicated objects in detail about their recursive relationships etc. O b je c t D ia g r a m O b je c t D ia g r a m

C la s s D ia g r a m

o b je c t n a m e

p h y s ic s M a t h s : D e p a r t m e n t

C la s s n a m e

1 D e p a rtm e n t

im p r o v e c la r it y

0 ..*

p h y s ic s : D e p a r t m e n t

m a th : D e p a r tm e n t

p u re M a th : D e p a rtm e n t

R e c u r s iv e c la s s d ia g r a m d if f ic u lt t o f u lly u n d e r s ta n d

a p p lie d M a t h : D e p a r t m e n t

S h o w s t h e d e t a ils o f t h e r e c u r s iv e o b je c t r e la t io n s h ip

When to use object diagrams? ƒ ƒ

Object diagrams are a vital for large projects. They are useful for explaining structural relationships in detail for complex objects.

Sequence diagrams: Sequence diagrams are interaction diagrams which detail what messages are sent and when. The sequence diagrams are organized according to time. The time progresses as you move from top to bottom of the diagram. The objects involved in the diagram are shown from left to right according to when they take part. S e q u e n c e D ia g r a m

c lie n t

a n O rd e r : O rd e r

m a k e A n O rd e r()

a n E n try : O r d e r E n tr y c h e c k if s u ffic ie n t d e ta ils a r e a v a ila b le fo r e a c h lin e I te m

m a k e A n O rd e r()

it e r a t io n [ fo r e a c h ...] ()

h a s S u f f i c i e n t D e t a i ls ( ) c o n fir m : C o n f ir m a tio n p r in tC o n fir m a tio n ( )

N o te : E a c h v e r t ic a l d o tt e d lin e is a l if e l in e . E a c h a r r o w is a m e s s a g e . T h e r e c t a n g u la r b o x e s o n t h e lif e lin e a r e c a lle d t h e a c t iv a t i o n b a r w h ic h r e p r e s e n ts t h e d u r a t io n o f e x e c u t io n o f m e s s a g e .

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Collaboration diagrams: Collaboration diagrams are also interaction diagrams. Collaboration diagrams convey the same message as the sequence diagrams. But the collaboration diagrams focus on the object roles instead of the times at which the messages are sent. The collaboration diagrams use the decimal sequence numbers as shown in the diagram below to make it clear which operation is calling which other operation, although it can be harder to see the overall sequence. The toplevel message is numbered 1. The messages at the same level have the same decimal prefix but different suffixes of 1, 2 etc according to when they occur.

Collaboration Diagram client 1.1 : makeAnOrder()

object

message

sequence number

1.1.1: makeAnOrder()

anOrder : Order

*

1.1.1.2 : hasSufficientDetails()

anEntry : OrderEntry

confirm : Confirmation

1 self-link

1.1.1.1: for each (iteration)

When to use interaction diagrams? ƒ

When you want to look at behaviour of several objects within a single use case. If you want to look at a single object across multiple use cases then use statechart diagram as described below.

State chart diagrams: Objects have behaviour and state. The state of an object is depends on its current activity or condition. This diagram shows the possible states of the object and the transitions that cause a change in its state. S t a t e c h a r t D ia g r a m

/ g e t a n it e m

/ a ll ite m s a v a ila b le C h e c k in g s ta te

D is p a tc h in g s t a t e

/ s o m e it e m s n o t in s t o c k

/ d e liv e r

/ it e m s r e c e iv e d w a itin g s t a t e

D e liv e r e d s t a t e

When to use statechart diagram? ƒ

Statechart diagrams are good at describing the behaviour of an object across several use cases. But they are not good at describing the interaction or collaboration between many objects. Use interaction and/or activity diagrams in conjunction with a statechart diagram.

ƒ

Use it only for classes that have complex state changes and behaviour. For example: the User Interface (UI) control objects, Objects shared by multi-threaded programs etc.

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Activity diagram: This is really a fancy flow chart. The activity diagram and statechart diagrams are related in a sense that statechart diagram focuses on object undergoing a transition process and an activity diagram focuses on the flow of activities involved in a single transition process. Activity Diagram O rder placem ent departm ent

swim lane

dispatch departm ent

start

check next item

get first item

activity

join

/ A ll item s in stock dispatch all item s / som e item s not in stock

W ait for item s to arrive in stock

place an order with the supplier

receive the order and enter into stock

In domain modelling it is imperative that the diagram conveys which object (or class) is responsible for each activity. Activity diagrams can be divided into object swimlanes that determine which object is responsible for which activity. The swimlanes are quite useful because they combine the activity diagram’s depiction of logic with the interaction diagram’s depiction of responsibility. A single transition comes out of each activity, connecting to the next activity. A transition may join or fork. When to use activity diagrams? The activity and statechart diagrams are generally useful to express complex operations. The great strength of activity diagrams is that they support and encourage parallel behaviour. The activity and statechart diagrams are beneficial for workflow modelling with multi-threaded programming. Component and Deployment diagrams: A component is a code module. Component diagrams are physical diagrams analogous to a class diagram. The deployment diagrams show the physical configuration of software and hardware components. The physical hardware is made up of nodes. Each component belongs to a node. Component and Deployment Diagram Ratailer J2EE Server

Order Component

OrderIF

Wholesaler J2EE application server

Dispatch Component

DispathIF

Order Component

OrderIF

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Q 108: What is the difference between aggregation and composition? SD DC A 108: Aggregation

Composition

Aggregation: An association in which one class belongs to another class or a collection. This is a part of a whole relationship where the part can exist without the whole. For example: A line item is whole and the products are the parts. If a line item is deleted then the products need not be deleted. (no cascade delete in database terms)

Composition: An association in which one class belongs to another class a collection. This is a part of a whole relationship where the part cannot exist without the whole. If the whole is deleted then the parts are deleted. For example: An Order is a whole and the line items are the parts. If an order is deleted then all the line items should be deleted as well (i.e. cascade deletes in database terms).

Aggregations are not allowed to be circular.

In a garbage-collected language like Java, The whole has the responsibility of preventing the garbage collector to prematurely collect the part by holding reference to it.

Q 109: What is the difference between a collaboration diagram and a sequence diagram? SD DC A 109: You can automatically generate one from the other. Sequence Diagram

Collaboration Diagram

The emphasis is on the sequence.

The emphasis is on the object roles

Reference: The above section on RUP & UML is based on the book UML Distilled by Martin Fowler and Kendall Scott. If you would like to have a good understanding of UML & RUP, then this book is recommended.

Enterprise - Struts Struts is a Web-based user interface framework, which has been around for a few years. It is a matured and proven framework, which has been used in many J2EE projects. While Struts has been demonstrating its popularity, there is an emerging framework called JavaServer Faces (JSF) gaining lots of momentum and popularity. Like Struts, JSF provides Web application life cycle management through a controller servlet, and like Swing, JSF provides a rich component model complete with event handling and component rendering. So JSF can be considered as a combination of Struts frame work and Java Swing user interface framework. Refer Q19 – Q20 in Emerging Technologies/Frameworks section for JSF.

Q 110: Give an overview of Struts? SF DP A 110: Struts is a framework with set of cooperating classes, servlets and JSP tags that make up a reusable MVC 2 design. S T R U T S O v e r v ie w

H

q

t

tc h

TT

P

re

sp

on

se

V ie w ( JS P )

A c ti o n (c a lls b u s in e s s lo g ic )

3. Instantiate/ Set

H

2. D i sp a

s tr u ts c o n f ig .x m l

C lie n t (B R O W S E R ) 6.

F ro n t C o n tr o l l e r (S e rvle t )

4. Foprward

1.

T

TP

re

s ue

5 . g e t th r o u g h T a g

M odel (F o rm b e a n s)

ƒ Client (Browser): A request from the client browser creates an HTTP request. The Web container will respond to the request with an HTTP response, which gets displayed on the browser.

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ƒ Controller (ActionServlet class and Request Processor class): The controller receives the request from the browser, and makes the decision where to send the request based on the struts-config.xml. Design pattern: Struts controller uses the command design pattern by calling the Action classes based on the configuration file struts-config.xml and the RequestProcessor class’s process() method uses template method design pattern (Refer Q11 in How would you go about … section) by calling a sequence of methods like: •

processPath(request, response) Æ read the request URI to determine path element.

•

processMapping(request,response) Æ use the path information to get the action mapping

•

processRoles(request,respose,mapping) Æ Struts Web application security which provides an authorization scheme. By default calls request.isUserInRole(). For example allow /addCustomer action if the role is executive.

•

processValidate(request,response,form,mapping) Æ calls the vaildate() method of the ActionForm.

•

processActionCreate(request,response,mapping)Æ gets the name of the action class from the “type” attribute of the element.

•

processActionPerform(req,res,action,form,mapping) Æ This method calls the execute method of the Action class which is where business logic is written.

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Business Logic (Action class): The Servlet dispatches the request to Action classes, which act as a thin wrapper to the business logic (The actual business logic is carried out by either EJB session beans and/or plain Java classes). The action class helps control the workflow of the application. (Note: The Action class should only control the workflow and not the business logic of the application). The Action class uses the Adapter design pattern (Refer Q11 in How would you go about … section).

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ActionForm class: Java representation of HTTP input data. They can carry data over from one request to another, but actually represent the data submitted with the request.

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View (JSP): The view is a JSP file. There is no business or flow logic and no state information. The JSP should just have tags to represent the data on the browser.

ActionServlet class is the controller part of the MVC implementation and is the core of the framework. It processes user requests, determines what the user is trying to achieve according to the request, pulls data from the model (if necessary) to be given to the appropriate view, and selects the proper view to respond to the user. As discussed above ActionServlet class delegates the grunt of the work to the RequestProcessor and Action classes. The ActionForm class maintains the state for the Web application. ActionForm is an abstract class, which is subclassed for every input form model. The struts-config.xml file controls, which HTML form request maps to which ActionForm. The Action class is a wrapper around the business logic. The purpose of the Action class is to translate the HttpServletRequest to the business logic. To use the Action class, subclass and overwrite the execute() method. The actual business logic should be in a separate package or EJB to allow reuse of business logic in protocol independent manner (ie the business logic should be used not only by HTTP clients but also by WAP clients, EJB clients, Applet clients etc). The ExceptionHandler can be defined to execute when the Action class’s execute() method throws an Exception. For example

When an IOException is thrown then it will be handled by the execute() method of the my.ExceptionHandler class. The struts-config.xml configuration information is translated into ActionMapping, which are put into the ActionMappings collection. Further reading is recommended for more detailed understanding.

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Q 111: What is a synchronizer token pattern in Struts or how will you protect your Web against multiple submissions? DC DP

A 111: Web designers often face the situation where a form submission must be protected against duplicate or multiple submissions. This situation typically occurs when the user clicks on submit button more than once before the response is sent back or client access a page by returning to the previously book marked page. ƒ

The simplest solution that some sites use is that displaying a warning message “Wait for a response after submitting and do not submit twice.

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In the client only strategy, a flag is set on the first submission and from then onwards the submit button is disabled based on this flag. Useful in some situations but this strategy is coupled to the browser type and version etc.

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For a server-based solution the J2EE pattern synchroniser token pattern can be applied. The basic idea is to: 1.

Set a token in a session variable on the server side before sending the transactional page back to the client.

2.

The token is set on the page as a hidden field. On submission of the page first check for the presence of a valid token by comparing the request parameter in the hidden field to the token stored in the session. If the token is valid continue processing otherwise take other alternative action. After testing the token must be reset to null.

The synchroniser token pattern is implemented in Struts. How do we implement the alternate course of action when the second clicks on submit button will cancel the response from the first click. The thread for the first click still runs but has no means of sending the response back to the browser. This means the transaction might have gone through without notifying the user. The user might get the impression that transaction has not gone through. Struts support for synchronisation comes in the form of: ActionServlet.saveToken(HttpRequest) and ActionServlet.isTokenValid(HttpRequest) etc

Q 112: How do you upload a file in Struts? SF A 112: In JSP page set the code as shown below: CO Please select the file that you would like to upload:

In the FormBean set the code as shown below: public class FileForm extends ActionForm { private FormFile file; public void setFile(FormFile file){ this.file = file; } public FormFile getFile(){ return file; } }

Q 113: Are Struts action classes thread-safe? SF CI A 113: No. Struts action classes are not thread-safe. Struts action classes are cached and reused for performance optimization at the cost of having to implement the action classes in a thread-safe manner.

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Q 114: How do you implement internationalization in Struts? SF A 114: Internationalization is built into Struts framework. In the JSP page set the code as shown below: CO i18n

Now we need to create an application resource file named ApplicationResource.properties. page.title=Thank you for visiting!

Now in Italian, create an application resource file named ApplicationResource_it.properties. page.title=Grazie per la vostra visita!

Finally, add reference to the appropriate resource file in the struts-config.xml.

Q 115: What is an action mapping in Struts? How will you extend Struts? SF A 115: An action mapping is a configuration file (struts-config.xml) entry that, in general, associates an action name with an action. An action mapping can contain a reference to a form bean that the action can use, and can additionally define a list of local forwards that is visible only to this action. How will you extend Struts? Struts is not only a powerful framework but also very extensible. You can extend Struts in one or more of the following ways: PlugIn: Define your own PlugIn class if you want to execute some init() and destroy() methods during the application startup and shutdown respectively. Some services like loading configuration files, initialising applications like logging, auditing, etc can be carried out in the init() method. RequestProcessor: You can create your own RequestProcessor by extending the Struts RequestProcessor. For example you can override the processRoles(req, res, mapping) in your extended class if you want to query the LDAP server for the security authorization etc. ActionServlet: You can extend the ActionServlet class if you want to execute your business logic at the application startup or shutdown or during individual request processing. You should take this approach only when the above mentioned approaches are not feasible.

Q 116: What design patterns are used in Struts? DP A 116: Struts is based on model 2 MVC (Model-View-Controller) architecture. Struts controller uses the command design pattern (Refer Q11 in How would you go about section) and the action classes use the adapter design pattern. The process() method of the RequestProcessor uses the template method design pattern (Refer Q11 in How would you go about section). Struts also implement the following J2EE design patterns ƒ ƒ ƒ ƒ ƒ ƒ

Service to Worker (Refer Q25 in Enterprise section). Dispatcher View (Refer Q25 in Enterprise section). Composite View (Struts Tiles) (Refer Q25 in Enterprise section) Front Controller (Refer Q24 in Enterprise section). View Helper (Refer Q25 in Enterprise section). Synchronizer Token (Refer Q111 in Enterprise section).

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Enterprise - Web and Application servers

Q 117: What application servers, Web servers, LDAP servers, and Database servers have you used? A 117: Web Servers

Apache, Microsoft IIS, Netscape, Domino etc

Application Servers

IBM Websphere, BEA Weblogic, Apache Tomcat, Borland Enterprise Server, Fujitsu Interstage, JBoss, ATG Dynamo etc

LDAP Servers

IPlanet’s directory server, SiemensDirX etc

Database Servers

IBM DB2, Oracle, SQL Server, Sybase, Informix

Q 118: What is the difference between a Web server and an application server? SF A 118: In general, an application server prepares data for a Web server -- for example, gathering data from databases, applying relevant business rules, processing security checks, and/or storing the state of a user’s session. The term application server may be misleading since the functionality isn’t limited to applications. Its role is more as retriever and manager of data and processes used by anything running on a Web server. In the coming age of Web services, application servers will probably have an even more important role in managing service oriented components. One of the reasons for using an application server is to improve performance by off-loading tasks from a Web server. When heavy traffic has more users, more transactions, more data, and more security checks then more likely a Web server becomes a bottleneck. Web Server

Application Server

Supports HTTP protocol. When a Web server receives an HTTP request, it responds with an HTTP response, such as sending back an HTML page (static content) or delegates the dynamic response generation to some other program such as CGI scripts or Servlets or JSPs in an application server.

Exposes business logic and dynamic content to a client through various protocols such as HTTP, TCP/IP, IIOP, JRMP etc.

Uses various scalability and fault-tolerance techniques.

Uses various scalability and fault-tolerance techniques. In addition provides resource pooling, component life cycle management, transaction management, messaging, security etc.

Q 119: What is a virtual host? SF A 119: The term virtual host refers to the practice of maintaining more than one server on one machine. They are differentiated by their host names. You can have name based virtual hosts and IP address based virtual hosts. For example A name-based "virtual host" has a unique domain name, but the same IP address. For example, www.company1.com and www.company2.com can have the same IP address 192.168.0.10 and share the same Web server. We can configure the Web server as follows: NameVirtualHost 192.168.0.10 ServerName www.company1.com DocumentRoot /web/company1 ServerName www.company2.com DocumentRoot /web/company2

In this scenario, both www.company1.com and www.company2.com are registered with the standard domain name service (DNS) registry as having the IP address 192.168.0.10. A user types in the URL http://www.company1.com/hello.jsp in their browser. The user's computer resolves the name

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www.company1.com to the IP address 192.168.0.10. The Web server on the machine that has the IP address 192.168.0.10, so it receives the request. The Web server determines which virtual host to use by matching the request URL It gets from an HTTP header submitted by the browser with the “ServerName” parameter in the configuration file shown above. Name-based virtual hosting is usually easier, since you have to only configure your DNS server to map each hostname to a single IP address and then configure the Web server to recognize the different hostnames as discussed in the previous paragraph. Name-based virtual hosting also eases the demand for scarce IP addresses limited by physical network connections [but modern operation systems supports use of virtual interfaces, which are also known as IP aliases]. Therefore you should use name-based virtual hosting unless there is a specific reason to choose IP-based virtual hosting. Some reasons why you might consider using IPbased virtual hosting: ƒ

Name-based virtual hosting cannot be used with SSL based secure servers because of the nature of the SSL protocol.

ƒ

Some operating systems and network equipment implement bandwidth management techniques that cannot differentiate between hosts unless they are on separate IP addresses.

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IP based virtual hosts are useful, when you want to manage more than one site (like live, demo, staging etc) on the same server where hosts inherit the characteristics defined by your main host. But when using SSL for example, a unique IP address is necessary.

For example in development environment when using the test client and the server on the same machine we can define the host file as shown below: UNIX user: /etc/hosts Windows user: C:\WINDOWS\SYSTEM32\DRIVERS\ETC\HOSTS 127.0.0.1 127.0.0.1 127.0.0.1

localhost www.company1.com www.company2.com

[Reference: http://httpd.apache.org/docs/1.3/vhosts/]

Q 120: What is application server clustering? SI A 120: An application server cluster consists of a number of application servers loosely coupled on a network. The server cluster or server group is generally distributed over a number of machines or nodes. The important point to note is that the cluster appears as a single server to its clients. The goals of application server clustering are: ƒ

Scalability: should be able to add new servers on the existing node or add new additional nodes to enable the server to handle increasing loads without performance degradation, and in a manner transparent to the end users.

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Load balancing: Each server in the cluster should process a fair share of client load, in proportion to its processing power, to avoid overloading of some and under utilization of other server resources. Load distribution should remain balanced even as load changes with time.

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High availability: Clients should be able to access the server at almost all times. Server usage should be transparent to hardware and software failures. If a server or node fails, its workload should be moved over to other servers, automatically as fast as possible and the application should continue to run uninterrupted. This method provides a fair degree of application system fault-tolerance. After failure, the entire load should be redistributed equally among working servers of the system.

[Good read: Uncover the hood of J2EE clustering by Wang Yu on http://www.theserverside.com ]

Q 121: Explain Java Management Extensions (JMX)? SF A 121: JMX framework can improve the manageability of your application by

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Monitoring your application for performance problems, critical events, error condition statistics, etc. For example you can be notified if there is a sudden increase in traffic or sudden drop in performance of your website.

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Making your application more controllable and configurable at runtime by directly exposing application API and parameters. For example you could switch your database connection to an alternate server. You can also change the level of debugging and logging within the application without stopping the server.

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By interfacing JMX to your hardware, database server and application server, health checks can be performed of your infrastructure.

Q 122: Explain some of the portability issues between different application servers? SI A 122: Transaction isolation levels, lazy loading and dirty marker strategies for EJB, class loading visibility etc.

Enterprise - Best practices and performance considerations

Q 123: Give some tips on J2EE application server performance tuning? PI A 123: ƒ

Set the Web container threads, which will be used to process incoming HTTP requests. The minimum size should be tuned to handle the average load of the container and maximum should be tuned to handle the peak load. The maximum size should be less than or equal to the number of threads in your Web server.

ƒ

When an EJB is called from a servlet or another EJB within the same JVM (i.e. same application server) then performance can be improved by running EJBs in pass-by-reference mode as oppose to pass-byvalue which is the default mode. Care should be taken to test the application properly before going into production because some valid applications may not work correctly when pass-by-reference setting is switched on.

ƒ

Application servers maintain a pool of JDBC resources so that a new connection does not need to be created for each transaction. Application servers can also cache your prepared statements to improve performance. So you can tune the minimum and maximum size of these pools.

ƒ

Tune your initial heap size for the JVM so that the garbage collector runs at a suitable interval so that it does not cause any unnecessary overhead. Adjust the value as required to improve performance.

ƒ

Set the session manager settings appropriately based on following guidelines: ƒ ƒ ƒ ƒ

Set the appropriate value for in memory session count. Reduce the session size. Don’t enable session persistence unless required by your application. Invalidate your sessions when you are finished with them by setting appropriate session timeout.

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Calls to EJB from a separate JVM are handled by ORB (Object Request Broker). ORB uses a pool of threads to handle these requests. The thread pool size should be set appropriately to handle average and peak loads.

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If a servlet or JSP file is called frequently with identical URL parameters then they can be dynamically cached to improve performance.

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Turn the application server tracing off unless required for debugging.

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Some application servers support lazy loading and dirty marker strategies with EJB to improve performance.

Q 124: Explain some of the J2EE best practices? BP A 124:

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Recycle your valuable resources by either pooling or caching. You should create a limited number of resources and share them from a common pool (e.g. pool of threads, pool of database connections, pool of objects etc). Caching is simply another type of pooling where instead of pooling a connection or object, you are pooling remote data (database data) and placing it in the memory (using Hashtable etc).

ƒ

Avoid embedding business logic in a protocol dependent manner like in JSPs, HttpServlets, Struts action classes etc. This is because your business logic should be not only executed by your Web clients but also required to be shared by various GUI clients like Swing based stand alone application, WAP clients etc.

ƒ

Automate the build process with tools like Ant, CruiseControl, and Maven etc. In an enterprise application the build process can become quite complex and confusing.

ƒ

Build test cases first (i.e. Test Driven Development (TDD), refer section Emerging Technologies) using tools like JUnit. Automate the testing process and integrate it with build process.

ƒ

Separate HTML code from the Java code: Combining HTML and Java code in the same source code can make the code less readable. Mixing HTML and scriplet will make the code extremely difficult to read and maintain. The display or behaviour logic can be implemented as a custom tags by the Java developers and Web designers can use these Tags as the ordinary XHTML tags.

ƒ

It is best practice to use multi-threading and stay away from single threaded model of the servlet unless otherwise there is a compelling reason for it. Shared resources can be synchronized or used in read-only manner or shared values can be stored in a database table. Single threaded model can adversely affect performance.

ƒ

Apply the following JSP best practices: ƒ

Place data access logic in JavaBeans: The code within the JavaBean is readily accessible to other JSPs and Servlets.

ƒ

Factor shared behaviour out of Custom Tags into common JavaBeans classes: The custom tags are not used outside JSPs. To avoid duplication of behaviour or business logic, move the logic into JavaBeans and get the custom tags to utilize the beans.

ƒ

Choose the right “include” mechanism: What are the differences between static and a dynamic include? Using includes will improve code reuse and maintenance through modular design. Which one to use? Refer Q31 in Enterprise section.

ƒ

Use style sheets (e.g. css), template mechanism (e.g. struts tiles etc) and appropriate comments (both hidden and output comments).

ƒ

If you are using EJBs apply the EJB best practices as described in Q82 in Enterprise section.

ƒ

Use the J2EE standard packaging specification to improve portability across Application Servers.

ƒ

Use proven frameworks like Struts, Spring, Hibernate, JSF etc.

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Apply appropriate proven J2EE design patterns to improve performance and minimise network communications cost (Session façade pattern, Value Object pattern etc).

ƒ

Batch database requests to improve performance. For example Connection con = DriverManager.getConnection(……). Statement stmt = con.createStatement(). stmt.addBatch(“INSERT INTO Address…………”); stmt.addBatch(“INSERT INTO Contact…………”); stmt.addBatch(“INSERT INTO Personal”); int[] countUpdates = stmt.executeBatch();

Use “PreparedStatements” instead of ordinary “Statements” for repeated reads. ƒ

Avoid resource leaks by ƒ ƒ

Closing all database connections after you have used them. Clean up objects after you have finished with them especially when an object having a long life cycle refers to a number of objects with short life cycles (you have the potential for memory leak).

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Poor exception handling where the connections do not get closed properly and clean up code that never gets called. You should put clean up code in a finally {} block.

Handle and propagate exceptions correctly. Decide between checked and unchecked (i.e RunTime exceptions) exceptions.

Q 125: Explain some of the J2EE best practices to improve performance? BP PI A 125: In short manage valuable resources wisely and recycle them where possible, minimise network overheads and serialization cost, and optimise all your database operations. ƒ

Manage and recycle your valuable resources by either pooling or caching. You should create a limited number of resources and share them from a common pool (e.g. pool of threads, pool of database connections, pool of objects etc). Caching is simply another type of pooling where instead of pooling a connection or object, you are pooling remote data (database data), and placing it in memory (using Hashtable etc). Unused stateful session beans must be removed explicitly and appropriate idle timeout should be set to control stateful session bean life cycle.

ƒ

Use effective design patterns to minimise network overheads (Session facade, Value Object etc Refer Q84, Q85 in Enterprise section), use of fast-lane reader pattern for database access (Refer Q86 in Enterprise section). Caching of retrieved JNDI InitialContexts, factory objects (e.g. EJB homes) etc. using the service locator design pattern, which reduces expensive JNDI access with the help of caching strategies.

ƒ

Minimise serialization costs by marking references (like file handles, database connections etc), which do not required serialization by declaring them ‘transient’ (Refer Q19 in Java section). Use pass-by-reference where possible as opposed to pass by value.

ƒ

Set appropriate timeouts: for the HttpSession objects, after which the session expires, set idle timeout for stateful session beans etc.

ƒ

Improve the performance of database operations with the following tips: ƒ

Database connections should be released when not needed anymore, otherwise there will be potential resource leakage problems.

ƒ

Apply least restrictive but valid transaction isolation level.

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Use JDBC prepared statements for overall database efficiency and for batching repetitive inserts and updates. Also batch database requests to improve performance.

ƒ

When you first establish a connection with a database by default it is in auto-commit mode. For better performance turn auto-commit off by calling the connection.setAutoCommit(false) method.

ƒ

Where appropriate (you are loading 100 objects into memory but use only 5 objects) lazy load your data to avoid loading the whole database into memory using the virtual proxy pattern. Virtual proxy is an object, which looks like an object but actually contain no fields until when one of its methods is called does it load the correct object from the database.

ƒ

Where appropriate eager load your data to avoid frequently accessing the database every time over the network.

Enterprise – Logging, testing and deployment

Q 126: Give an overview of log4J? SF A 126: Log4j is a logging framework for Java. Log4J is designed to be fast and flexible. Log4J has 3 main components which work together to enable developers to log messages: ƒ ƒ ƒ

Loggers [was called Category prior to version 1.2] Appenders Layout

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Logger: The foremost advantage of any logging API like log4J, apache commons logging etc over plain System.out.println is its ability to disable certain log statements while allowing others to print unhindered. Loggers are hierarchical. The root logger exists at the top of the hierarchy. The root logger always exists and it cannot be retrieved by name. The hierarchical nature of the logger is denoted by “.” notation. For example the logger “java.util” is the parent of child logger “java.util.Vector” and so on. Loggers may be assigned priorities such as DEBUG, INFO, WARN, ERROR and FATAL. If a given logger is not assigned a priority, then it inherits the priority from its closest ancestor. The logging requests are made by invoking one of the following printing methods of the logger instance: debug(), info(), warn(), error(), fatal(). Appenders and Layouts: In addition to selectively enabling and disabling logging requests based on the logger, the log4J allows logging requests to multiple destinations. In log4J terms the output destination is an appender. There are appenders for console, files, remote sockets, JMS, etc. One logger can have more than one appender. A logging request for a given logger will be forwarded to all the appenders in that logger plus the other appenders higher in the hierarchy. In addition to the output destination the output format can be categorised as well. This is accomplished by associating layout with an appender. The layout is responsible for formatting the logging request according to user’s settings. Sample configuration file: #set the root logger priority to DEBUG and its appender to App1 log4j.rootLogger=DEBUG, App1 #App1 is set to a console appender log4j.appender.App1=org.apache.log4j.ConsoleAppender #appender App1 uses a pattern layout log4j.appender.App1.layout=org.apache.log4j.PatternLayout. log4j.appender.App1.layout.ConversionPattern=%-4r [%t] %-5p %c %x -%m%n # Print only messages of priority WARN or above in the package com.myapp log4j.Logger.com.myapp=WARN

XML configuration for log4j is available, and is usually the best practise.

Q 127: How do you initialize and use Log4J? SF CO A 127: public class MyApp { //Logger is a utility wrapper class to be written with appropriate printing methods static Logger log = Logger.getLogger (MyApp.class.getName()); public void my method() { if(log.isDebugEnabled()) log.debug(“This line is reached………………………..” + var1 + “-” + var2); ) } }

Q 128: What is the hidden cost of parameter construction when using Log4J? SF PI A 128: Do not use in frequently accessed methods or loops: CO log.debug (“Line number” + intVal + “ is less than ” + String.valueOf(array[i]));

The above construction has a performance cost in frequently accessed methods and loops in constructing the message parameter, concatenating the String etc regardless of whether the message will be logged or not. Do use in frequently accessed methods or loops: CO If (log.isDebugEnabled()) { log.debug (“Line number” + intVal + “ is less than ” + String.valueOf(array[i])); }

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The above construction will avoid the parameter construction cost by only constructing the message parameter when you are in debug mode. But it is not a best practise to place log.isDebugEnabled() around all debug code.

Q 129: What is the test phases and cycles? SD A 129: ƒ

Unit tests (e.g. JUnit etc, carried out by developers). There are two popular approaches to testing server-side classes: mock objects, which test classes by simulating the server container, and in-container testing, which tests classes running in the actual server container. If you are using Struts framework, StrutsTestCase for JUnit allows you to use either approach, with very minimal impact on your actual unit test code.

ƒ ƒ ƒ ƒ ƒ

System tests or functional tests (carried out by business analysts and/or testers). Integration tests (carried out by business analysts, testers, developers etc). Regression tests (carried out by business analysts and testers). Stress volume tests or load tests (carried out by technical staff). User acceptance tests (UAT – carried out by end users).

Each of the above test phases will be carried out in cycles. Refer Q13 in How would you go about… section for JUnit, which is an open source unit-testing framework.

Q 130: Brief on deployment environments you are familiar with? A 130: Differ from project team to project team [Hint] : Application environments where “ear” files get deployed. Development box: can have the following instances of environments in the same machine (need not be clustered). ƒ ƒ

Development environment Æ used by developers. System testing environment Æ used by business analysts.

Staging box: can have the following instances of environments in the same machine (preferably clustered servers with load balancing) ƒ ƒ

Integration testing environment Æ used for integration testing, user acceptance testing etc. Pre-prod environment Æ used for user acceptance testing, regression testing, and load testing or stress volume testing (SVT). [This environment should be exactly same as the production environment].

Production box: ƒ

Production environment Æ live site used by actual users.

Data environments (Database) Note: Separate boxes [not the same boxes as where applications (i.e. ear files) are deployed] ƒ

Development box (database). Used by applications on development and system testing environments. Separate instances can be created on the same box for separate environments like development and system testing.

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Staging Box (database) Used by applications on integration testing and user acceptance testing environments. Separate instances can be created on the same box for separate environments.

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Production Box (database) Live data used by actual users of the system.

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Enterprise - Personal

Q 131: Tell me about yourself or about some of the recent projects you have worked with? What do you consider your most significant achievement? Why do you think you are qualified for this position? Why should we hire you and what kind of contributions will you make? A 131: [Hint:] Pick your recent projects and give a brief overview of it. Also it is imperative that during your briefing that you demonstrate how you applied your skills and knowledge in some of the following key areas and fixed any issues. ƒ ƒ ƒ ƒ ƒ ƒ ƒ ƒ ƒ ƒ

Design Concepts: Refer Q02, Q03, Q19, Q20, Q21, Q91, Q98, and Q101. Design Patterns: Refer Q03, Q24, Q25, Q83, Q84, Q85, Q86, Q87, Q88 and Q111. Performance issues: Refer Q10, Q16, Q45, Q46, Q97, Q98, Q100, Q123, and Q125. Memory issues: Refer Q45 and Q93 Multi-threading (Concurrency issues): Refer Q16, Q34, and Q113 Exception Handling: Refer Q76 and Q77 Transactional issues: Refer Q43, Q71, Q72, Q73, Q74, Q75 and Q77. Security issues: Refer Q23, Q58, and Q81 Scalability issues: Refer Q20, Q21, Q120 and Q122. Best practices: Refer Q10, Q16, Q39, Q40, Q46, Q82, Q124, and Q125

Refer Q66 – Q72 in Java section for frequently asked non-technical questions.

Q 132: Have you used any load testing tools? A 132: Rational Robot, JMeter, LoadRunner, etc. Q 133: What source control systems have you used? SD A 133: CVS, VSS (Visual Source Safe), Rational clear case etc. Refer Q13 in How would you go about section…. for CVS.

Q 134: What operating systems are you comfortable with? SD A 134: NT, Unix, Linux, Solaris etc Q 135: Which on-line technical resources do you use to resolve any design and/or development issues? A 135: http://www.theserverside.com, http://www.javaworld.com, http://www-136.ibm.com/developerworks/Java/, http://java.sun.com/, www.javaperformancetuning.com etc

Enterprise – Software development process

Q 136: What software development processes/principles are you familiar with? SD A 136: Agile (i.e. lightweight) software development process is gaining popularity and momentum across organizations. Agile software development manifesto Æ [Good read: http://www.agilemanifesto.org/principles.html]. ƒ Highest priority is to satisfy the customer. ƒ Welcome requirement changes even late in development life cycle. ƒ Business people and developers should work collaboratively. ƒ Form teams with motivated individuals who produce best designs and architectures.

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ƒ Teams should be pro-active on how to become more effective without becoming complacent. ƒ Quality working software is the primary measure of progress. Why is iterative development with vertical slicing used in agile development? Your overall software quality can be improved through iterative development, which provides you with constant feedback. Traditional Vs Agile approach Traditional approach With the tradional approach, Say for example w e have a fundamental flaw in the data layer, if this flaw gets only picked up after the milestone 3, then there w ill be lot of rew ork to be done to the business and the presentation layer. This is the major draw back w ith the traditional development approach w here there is no vertical slicing.

Data Layer

Business Layer

Data Layer

milestone 3

Business Layer

milestone 2

Data Layer

milestone 1

technical scope

Presentation Layer

project tim e

Data Layer

Business layesr Data Layer

Presentation Layer iteration 2

Busines s layesr

Presentatio n Layer

Business Layer

Data Layer

As you can see w ith the agileiterative approach, a vertical slice is built for each iteration. So any fundamental flaw in design or coding can be picked up early and rectified. Even deployment and testing w ill be carried out in vertical slices.

iteration 3

Present ation Layer

iteration 1

technical scope

Agile (light w eight )approach

project tim e

Several methodologies fit under this agile development methodology banner. All these methodologies share many characteristics like iterative and incremental development, test driven development, stand up meetings to improve communication, automatic testing, build and continuous integration of code etc. Among all the agile methodologies XP is the one which has got the most attention. Different companies use different flavours of agile methodologies by using different combinations of methodologies. How does vertical slicing influence customer perception? With the iterative and incremental approach, customer will be comfortable with the progress of the development as opposed to traditional big bang approach. Tradional Vs Agile pe rce iv e d functionality

Traditional

A s f ar as the developer is concerned 65% of coding has been completed but f rom the customer's view only 20% of the f unctionality has been completed

Presentation Layer

Agile

A s f ar as the developer is concerned 65% of coding has been completed and f rom the customer's view 65% of the f unctionality has been completed. So the customer is happy.

Presentation Layer

Business Layer

Business Layer

Data Layer

Data Layer

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EXtreme Programming [XP] Æ simple design, pair programming, unit testing, refactoring, collective code ownership, coding standards, etc. Refer Q10 in “How would you go about…” section. XP has four key values: Communication, Feedback, Simplicity and Courage. It then builds up some tried and tested practices and techniques. XP has a strong emphasis on testing where tests are integrated into continuous integration and build process, which yields a highly stable platform. XP is designed for smaller teams of 20 – 30 people.

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RUP (Rational Unified Process) Æ Model driven architecture, design and development; customizable frameworks for scalable process; iterative development methodology; Re-use of architecture, code, component, framework, patterns etc. RUP can be used as an agile process for smaller teams of 20-30

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people, or as a heavy weight process for larger teams of 50-100 people. Refer Q103 – Q105 in Enterprise section. ƒ

Feature Driven Development [FDD] Æ Jeff De Luca and long time OO guru Peter Coad developed feature Driven Development (FDD). Like the other adaptive methodologies, it focuses on short iterations that deliver tangible functionality. FDD was originally designed for larger project teams of around 50 people. In FDD's case the iterations are two weeks long. FDD has five processes. The first three are done at the beginning of the project. The last two are done within each iteration. 1. 2. 3. 4. 5.

Develop an Overall Model Build a Features List Plan by Feature Design by Feature Build by Feature

The developers come in two kinds: class owners and chief programmers. The chief programmers are the most experienced developers. They are assigned features to be built. However they don't build them alone. Instead the chief programmer identifies which classes are involved in implementing the feature and gathers their class owners together to form a feature team for developing that feature. The chief programmer acts as the coordinator, lead designer, and mentor while the class owners do much of the coding of the feature. ƒ

Test Driven Development [TDD] Æ TDD is an iterative software development process where you first write the test with the idea that it must fail. Refer Q1 in Emerging Technologies/Frameworks section…

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Scrum Æ Scrum divides a project into sprints (aka iterations) of 30 days. Before you begin a sprint you define the functionality required for that sprint and leave the team to deliver it. But every day the team holds a short (10 – 15 minute) meeting, called a scrum where the team runs through what it will achieve in the next day. Some of the questions asked in the scrum meetings are: ƒ ƒ ƒ

What did you do since the last scrum meetings? Do you have any obstacles? What will you do before next meeting?

This is very similar to stand-up meetings in XP and iterative development process in RUP.

Enterprise – Key Points

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J2EE is a 3-tier (or n-tier) system. Each tier is logically separated and loosely coupled from each other, and may be distributed.

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J2EE applications are developed using MVC architecture, which divides the functionality of displaying and maintaining of the data to minimise the degree of coupling between enterprise components.

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J2EE modules are deployed as ear, war and jar files, which are standard application deployment archive files.

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HTTP is a stateless protocol and state can be maintained between client requests using HttpSession, URL rewriting, hidden fields and cookies. HttpSession is the recommended approach.

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Servlets and JSPs are by default multi-threaded, and care should be taken in declaring instance variables and accessing shared resources. It is possible to have a single threaded model of a servlet or a JSP but this can adversely affect performance.

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Clustering promotes high availability and scalability. The considerations for servlet clustering are: ƒ ƒ ƒ ƒ ƒ

Objects stored in the session should be serializable. Design for idempotence. Avoid using instance and static variables in read and write mode. Avoid storing values in the ServletContext. Avoid using java.io.* and use getResourceAsStream() instead.

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JSPs have a translation or a compilation process where the JSP engine translates and compiles a JSP file into a JSP servlet.

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JSPs have 4 different scope values: page, request, session and application. JSPs can be included statically, where all the included JSP pages are compiled into a single servlet during the translation or compilation phase or included dynamically, where included JSPs are compiled into separate servlets and the content generated by these servlets are included at runtime in the JSP response.

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Avoid scriptlet code in your JSPs and use JavaBeans or custom tags (e.g. Struts tags, JSTL tags, JSF tags etc) instead.

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Databases can run out cursors if the connections are not closed properly. The valuable resources like connections and statements should be enclosed in a try{} and finally{} block.

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Prepared statements offer better performance as opposed to statements, as they are precompiled and reuse the same execution plan with different arguments. Prepared statements are also more secure because they use bind variables, which can prevent SQL injection attacks.

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JNDI provides a generic interface to LDAP and other directory services like NDS, DNS etc.

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In your code always make use of a logical JNDI reference (java:comp/env/ejb/MyBean) as opposed to physical JNDI reference (ejb/MyBean) because you cannot guarantee that the physical JNDI location you specify in your code will be available. Your code will break if the physical location is changed.

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LDAP servers are typically used in J2EE applications to authenticate and authorise users. LDAP servers are hierarchical and are optimized for read access, so likely to be faster than database in providing read access.

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RMI facilitates object method calls between JVMs. JVMs can be located on separate host machines, still one JVM can invoke methods belonging to an object residing in another JVM (i.e. address space). RMI uses object serialization to marshal and unmarshal parameters. The remote objects should extend the UnicastRemoteObject.

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To go through a firewall, the RMI protocol can be embedded within the firewall trusted HTTP protocol, which is called HTTP tunnelling.

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EJB (i.e. 2.x) is a remote, distributed multi-tier system, which supports protocols like JRMP, IIOP, and HTTP etc. EJB components contain business logic and system level supports like security, transaction, instance pooling, multithreading, object life-cycles etc are managed by the EJB container and hence simplify the programming effort. Having said this, there are emerging technologies like: ƒ ƒ

Hibernate, which is an open source object-to-relational (O/R) mapping framework. EJB 3.0, which is taking ease of development very seriously and has adjusted its model to offer the plain old Java objects (i.e. POJOs) based persistence and the new O/R mapping model based on hibernate.

Refer Q14 – Q18 in Emerging technologies / Frameworks section for brief discussion on hibernate and EJB 3.0. ƒ

EJB transaction attributes (like Required, Mandatory, RequiresNew, Supports etc) are specified declaratively through EJB deployment descriptors. Isolation levels are not part of the EJB 2.x specification. So the isolation levels can be set on the resource manager either explicitly on the Connection or via the application server specific configuration.

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A transaction is often described by ACID (Atomic, Consistent, Isolated and Durable) properties. A distributed transaction is an ACID transaction between two or more independent transactional resources like two separate databases. A 2-phase commit is an approach for committing a distributed transaction in 2 phases.

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EJB 2.x has two types of exceptions: ƒ ƒ

System exception: is an unchecked exception derived from java.lang.RuntimeException. It is thrown by the system and is not recoverable. Application exception: is specific to an application and is thrown because of violation of business rules.

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EJB container managed transactions are automatically rolled back when a system exception occurs. This is possible because the container can intercept system exceptions. However when an application exception occurs, the container does not intercept and leaves it to the code to roll back using ctx.setRollbackOnly() method.

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EJB containers can make use of lazy loading (i.e. not creating an object until it is accessed) and dirty marker (ie persist only the entity beans that have bean modified) strategies to improve entity beans performance.

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Message Oriented Middleware (MOM) is a software infrastructure that asynchronously communicates with other disparate systems through the production and consumption of messages. Messaging enables loosely coupled distributed communication. Java Messaging Service (JMS) is a Java API that allows applications to create, send, receive read messages in a standard way, hence improves portability.

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Some of the design decisions you need to make in JMS are message acknowledgement modes, transaction modes, delivery modes etc, synchronous vs. asynchronous paradigm, message body types, setting appropriate timeouts etc.

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XML documents can be processed in your Java/J2EE application either using a SAX parser, which is event driven or a DOM parser, which creates a tree structure in memory. The other XML related technologies are DTD, XSD, XSL, XPath, etc and Java and XML based technologies are JAXP, JAXB etc.

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There is an impedance mismatch between object and relational technology. Classes represent both data and behaviour whereas relational database tables just implement data. Inheritance class structure can be mapped to relational data model in one of the following ways: ƒ ƒ ƒ ƒ

Map class hierarchy to single database table. Map each class to its own table. Map each concrete class to its own table Generic meta-data driven approach.

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Normalize data in your database for accuracy and denormalize data in your database for performance.

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RUP (Rational Unified Process) has 4 phases in the following order Inception, Elaboration, Construction, and Transition. Agile (i.e. lightweight) software development process is gaining popularity and momentum across organizations. Several methodologies like XP, RUP, Scrum, FDD, TDD etc fit under this agile development methodology banner. All these methodologies share many characteristics like iterative and incremental development, stand-up meetings to improve communication, automatic build, testing and continuous integration etc.

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UML is applicable to the object oriented (OO) problem solving. There are different types of UML diagrams like use case diagrams, class diagrams, sequence diagrams, collaboration diagrams, state chart diagrams, activity diagrams, component diagrams, deployment diagrams etc.

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Class diagrams are vital within OO methods. Class diagrams have the following possible relationships, association, aggregation, composition, generalization, and dependency.

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Struts is an MVC framework. Struts action classes are not thread-safe and care should be taken in declaring instance variables or accessing other shared resources. JSF is another Web UI framework like Struts gaining popularity and momentum.

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Log4j has three main components: loggers, appenders and layouts. Logger is a utility wrapper class. JUnit is an open source unit-testing framework.

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You can improve the performance of a J2EE application as follows : 1. Manage and recycle your valuable resources like connections, threads etc by either pooling or caching. 2. Use effective design patterns like session façade, value object, fast lane reader etc to minimise network overheads. 3. Set appropriate timeouts for HttpSession objects. 4. Use JDBC prepared statements as opposed to statements. 5. Release database connections in a finally {} block when finished. 6. Apply least restrictive but valid transaction isolation level. 7. Batch database requests. 8. Minimise serialization costs by marking references like file handles, database connections, etc which do not require serialization by declaring them transient.

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Some of the J2EE best practices are: 1. 2. 3. 4. 5.

Recycle your valuable resources by either pooling or caching. Automate your build process with tools like Ant, CruiseControl, and Maven etc, and continuously integrate your code into your build process. Build test cases first using tools like JUnit. Use standard J2EE packaging to improve portability. Apply appropriate proven design patterns.

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Use proven frameworks like Struts, Spring, Hibernate, JSF, JUnit, Log4J, etc. Handle and propagate exceptions correctly. Avoid resource leaks by closing all database connections after you have used them.

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The goals of application server clustering are to achieve scalability, load balancing, and high availability.

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Java Management Extension (JMX) framework can improve the manageability of your application, for performance problems, critical events, error conditions etc and perform health checks on your hardware, database server etc. You can also configure and control your application at runtime.

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Finally get familiarised with some of the key Java & J2EE design patterns like: 1.

MVC design pattern: J2EE uses this design pattern or architecture.

2.

Chain of responsibility design pattern: Servlet filters use a slightly modified version of chain of responsibility design pattern.

3.

Front controller J2EE design pattern: provides a centralized access point for HTTP request handling to support the integration system services like security, data validation etc. This is a popular J2EE design pattern.

4.

Composite view J2EE design pattern: creates an aggregate view from atomic sub-views.

5.

View helper J2EE design pattern: avoids duplication of code. The helper classes are JavaBeans and custom tags (e.g. Struts tags, JSF tags, JSTL tags etc).

6.

Service to worker and dispatcher view J2EE design pattern: These two patterns are a combination of front controller and view helper patterns with a dispatcher component. These two patterns differ in the way they suggest different division of responsibility among components.

7.

Bridge design pattern: Java Data Base Connectivity (JDBC) uses the bridge design pattern. The JDBC API provides an abstraction and the JDBC drivers provide the implementation.

8.

Proxy design pattern: RMI & EJB uses the proxy design pattern. A popular design pattern.

9.

Business delegate J2EE design pattern: used to reduce the coupling between the presentation tier and the business services tier components.

10. Session façade J2EE design pattern: too many fine-grained method calls between the client and the server will lead to network overhead and tight coupling. Use a session bean as a façade to provide a coarse-grained service access layer to clients. 11. Value object J2EE design pattern: avoid fine-grained method calls by creating a value object, which will help the client, make a coarse-grained call. 12. Fast-lane reader J2EE design pattern: access the persistence layer directly using a DAO (Data Access Object) pattern instead of using entity beans. 13. Service locator J2EE design pattern: expensive and redundant JNDI lookups can be avoided by caching and reusing the already looked up service objects. Recommended reading on J2EE design patterns: ƒ Core J2EE Patterns: Best Practices and Design Strategies, Second Edition (Hardcover) by Deepak Alur, Dan Malks, John Crupi.

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Let us put all together in the next section

LF

DC

CI

PI

SE

EH

SD

DP

SF

MI

SI

TI

BP

CO

How would you go about…?

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SECTION THREE

How would you go about…?

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This section basically assesses your knowledge of how to perform certain tasks like documenting your project, identifying any potential performance, memory, transactional, and/or design issues etc.

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It also assesses if you have performed any of these tasks before. If you have not done a particular task, you can demonstrate that you know how to go about it if the task is assigned to you.

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This section also recaps some of the key considerations discussed in the Java and Enterprise sections. Question numbers are used for cross-referencing with Java and Enterprise sections.

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Q11 & Q13 are discussed in more detail and can be used as a quick reference guide in a software project. All the other questions excluding Q11 & Q13 can be read just before an interview.

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Q 01: How would you go about documenting your Java/J2EE application? A 01: To be successful with a Java/J2EE project, proper documentation is vital. ƒ

Before embarking on coding get the business requirements down. Build a complete list of requested features, sample screen shots (if available), use case diagrams, business rules etc as a functional specification document. This is the phase where business analysts and developers will be asking questions about user interface requirements, data tier integration requirements, use cases etc. Also prioritize the features based on the business goals, lead-times and iterations required for implementation.

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Prepare a technical specification document based on the functional specification. The technical specification document should cover: ™

Purpose of the document: This document will emphasise the customer service functionality …

™

Overview: This section basically covers background information, scope, any inclusions and/or exclusions, referenced documents etc.

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Basic architecture: discusses or references baseline architecture document. Answers questions like Will it scale? Can this performance be improved? Is it extendable and/or maintainable? Are there any security issues? Describe the vertical slices to be used in the early iterations, and the concepts to be proved by each slice. Etc. For example which MVC [model-1, model-2 etc] paradigms (Refer Q3 in Enterprise section for MVC) should we use? Should we use Struts, JSF, Spring etc or build our own framework? Should we use a business delegate (Refer Q83 in Enterprise section for business delegate) to decouple middle tier with the client tier? Should we use AOP (Aspect Oriented Programming) (Refer Q3 in Emerging Technologies/Frameworks)? Should we use dependency injection? Should we use annotations? Do we require internationalization? Etc.

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Assumptions, Dependencies, Risks and Issues: highlight all the assumptions, dependencies, risks and issues. For example list all the risks you can identify.

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Design alternatives for each key functional requirement. Also discuss why a particular design alternative was chosen over the others. This process will encourage developers analyse the possible design alternatives without having to jump at the obvious solution, which might not always be the best one.

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Processing logic: discuss the processing logic for the client tier, middle tier and the data tier. Where required add process flow diagrams. Add any pre-process conditions and/or post-process conditions. (Refer Q9 in Java section for design by contract).

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UML diagrams to communicate the design to the fellow developers, solution designers, architects etc. Usually class diagrams and sequence diagrams are required. The other diagrams may be added for any special cases like (Refer Q107 in Enterprise section):

™ ƒ

™

State chart diagram: useful to describe behaviour of an object across several usecases.

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Activity diagram: useful to express complex operations. Supports and encourages parallel behaviour. Activity and statechart diagrams are beneficial for workflow modelling with multi threaded programming.

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Collaboration and Sequence diagrams: Use a collaboration or sequence diagram when you want to look at behaviour of several objects within a single use case. If you want to look at a single object across multiple use cases then use statechart.

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Object diagrams: The Object diagrams show instances instead of classes. They are useful for explaining some complicated objects in detail such as highlighting recursive relationships etc.

List the package names, class names, database names and table names with a brief description of their responsibility in a tabular form.

Prepare a coding standards document for the whole team to promote consistency and efficiency. Some coding practices can degrade performance for example: ™

Inappropriate use of String class. Use StringBuffer instead of String for compute intensive mutations (Refer Q17 in Java section).

How would you go about…?

™

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Code in terms of interface. For example you might decide the LinkedList is the best choice for some application, but then later decide ArrayList might be a better choice. (Refer Q15 in Java section) Wrong approach Æ ArrayList list = new ArrayList(); Right approach Æ List list = new ArrayList(100)

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Set the initial capacity of a collection appropriately (e.g. ArrayList, HashMap etc). (Refer Q15 in Java section).

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To promote consistency define standards for variable names, method names, use of logging, curly bracket positions etc.

Prepare a code review document and templates for the whole team. Let us look at some of the elements the code review should cover: ™

Proper variable declaration: e.g. instance versus static variables, constants etc.

™

Performance issues: e.g. Use ArrayList, HashMap etc instead of Vector, Hashtable when there is no thread-safety issue.

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Memory issues: e.g. Improper instantiation of objects instead of object reuse and object pooling, not closing valuable resource in a finally block etc.

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Thread-safety issues: e.g. Java API classes like SimpleDateFormat, Calendar, DecimalFormat etc are not thread safe, declaring variables in JSP is not thread safe, storing state information in Struts action class or multi-threaded servlet is not thread safe.

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Error handling: e.g. Re-throwing exception without nesting original exception, EJB methods not throwing EJB exception for system exceptions, etc.

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Use of coding standards: e.g. not using frameworks, System.out is used instead of log4j etc.

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Design issues: No re-use of code, no clear separation of responsibility, invalid use of inheritance to get method reuse, servlets performing JDBC direct access instead of using DAO (Data Access Objects) classes, HTML code in Struts action or servlet classes, servlets used as utility classes rather than as a flow controller etc.

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Documentation of code: e.g. No comments, no header files etc

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Bugs: e.g. Calling setAutoCommit within container-managed transaction, binary OR “|” used instead of logical OR “||”, relying on pass-by-reference in EJB remote calls, ResultSet not being closed on exceptions, EJB methods not throwing EJBException for system exceptions etc (Refer Q76 & Q77 in Enterprise section)

Prepare additional optional guideline documents as per requirements to be shared by the team. This will promote consistency and standards. For example: ™ ™ ™ ™ ™ ™ ™ ™

Guidelines to setting up J2EE development environment. Guidelines to version control system (CVS, VSS etc). Guidelines to deployment steps, environment settings, ant targets etc. Guidelines for the data modelling (any company standards). Guidelines for error handling (Refer Q34, Q35 in Java section & Q76, Q77 in Enterprise section). Guidelines for user interface design. Project overview document. Software development process document etc.

Some of the above mentioned documents, which are shared by the whole team, can be published in an internal website like Wiki. Wiki is a piece of server software that allows users to freely create and edit Web page content using any Web browser.

Q 02: How would you go about designing a Java/J2EE application?

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A 02: Design should be specific to a problem but also should be general enough to address future requirements. Designing reusable object oriented software involves decomposing the business use cases into relevant objects and converting objects into classes. ƒ

Create a tiered architecture: client tier, business tier and data tier. Each tier can be further logically divided into layers (Refer Q2, Q3 on Enterprise section). Use MVC (Model View Controller architecture for the J2EE and Java based GUI applications).

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Create a data model: A data model is a detailed specification of data oriented structures. This is different from the class modelling because it focuses solely on data whereas class models allow you to define both data and behaviour. Conceptual data models (aka domain models) are used to explore domain concepts with project stakeholders. Logical data models are used to explore the domain concepts, and their relationships. Logical data models depict entity types, data attributes end entity relationships (with Entity Relationship (ER) diagrams). Physical data models are used to design the internal schema of a database depicting the tables, columns, and the relationships between the tables. Data models can be created by performing the following tasks: ™

Identify entity types, attributes and relationships: use entity relationship (E-R) diagrams.

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Apply naming conventions (e.g. for tables, attributes, indices, constraints etc): Your organization should have standards and guidelines applicable to data modelling.

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Assign keys: surrogate keys (e.g. assigned by the database like Oracle sequences etc, max()+1, universally unique identifiers UUIDs, etc), natural keys (e.g. Tax File Numbers, Social Security Numbers etc), and composite keys.

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Normalize to reduce data redundancy and denormalize to improve performance: Normalized data have the advantage of information being stored in one place only, reducing the possibility of inconsistent data. Furthermore, highly normalized data are loosely coupled. But normalization comes at a performance cost because to determine a piece of information you have to join multiple tables whereas in a denormalized approach the same piece of information can be retrieved from a single row of a table. Denormalization should be used only when performance testing shows that you need to improve database access time for some of your tables.

Note: Creating a data model (logical, physical etc) before design model is a matter of preference, but many OO methodologies are based on creating the data model from the object design model (i.e. you may need to do some work to create an explicit data model but only after you have a complete OO domain and design model ). In many cases when using ORM tools like Hibernate, you do not create the data model at all.

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Create a design model: A design model is a detailed specification of the objects and relationships between the objects as well as their behaviour. (Refer Q107 on Enterprise section) ™

Class diagram: contains the implementation view of the entities in the design model. The design model also contains core business classes and non-core business classes like persistent storage, security management, utility classes etc. The class diagrams also describe the structural relationships between the objects.

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Use case realizations: are described in sequence and collaboration diagrams.

Design considerations when decomposing business use cases into relevant classes: designing reusable and flexible design models requires the following considerations: ™

Granularity of the objects (fine-grained, coarse-grained etc): Can we minimise the network trip by passing a coarse-grained value object instead of making 4 network trips with fine-grained parameters? (Refer Q85 in Enterprise section). Should we use method level (coarse-grained) or code level (finegrained) thread synchronization? (Refer Q40 in Java section). Should we use a page level access security or a fine-grained programmatic security?

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Coupling between objects (loosely coupled versus strongly coupled). Should we use business delegate pattern to loosely couple client and business tier? (Refer Q83 in Enterprise section) or Should we use dependency injection? (Refer Q09 in Emerging Technologies/Frameworks).

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Network overheads for remote objects like EJB, RMI etc: Should we use the session façade, value object patterns? (Refer Q84 & Q85 in Enterprise section).

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Definition of class interfaces and inheritance hierarchy: Should we use an abstract class or an interface? Is there any common functionality that we can move to the super class (or parent class)? Should we use interface inheritance with object composition for code reuse as opposed to implementation inheritance? Etc. (Refer Q8, Q10 in Java section).

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Establishing key relationships (aggregation, composition, association etc): Should we use aggregation or composition? [composition may require cascade delete] (Refer Q107, Q108 in Enterprise section – under class diagrams). Should we use an “is a” (generalization) relationship or a “has a” (composition) relationship? (Refer Q7 in Java section).

™

Applying polymorphism and encapsulation: Should we hide the member variables to improve integrity and security? (Refer Q8 in Java section). Can we get a polymorphic behaviour so that we can easily add new classes in the future? (Refer Q8 in Java section).

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Applying well-proven design patterns (like Gang of four design patterns, J2EE design patterns, EJB design patterns etc) help designers to base new designs on prior experience. Design patterns also help you to choose design alternatives (Refer Q11 in How would you go about…).

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Scalability of the system: Vertical scaling is achieved by increasing the number of servers running on a single machine. Horizontal scaling is achieved by increasing the number of machines in the cluster. Horizontal scaling is more reliable than the vertical scaling because there are multiple machines involved in the cluster. In vertical scaling the number of server instances that can be run on one machine are determined by the CPU usage and the JVM heap memory.

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How do we replicate the session state? Should we use stateful session beans or HTTP session? Should we serialize this object so that it can be replicated?

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Internationalization requirements for multi-language support: Should we support other languages? Should we support multi-byte characters in the database?

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Vertical slicing: Getting the reusable and flexible design the first time is impossible. By developing the initial vertical slice of your design you eliminate any nasty integration issues later in your project. Also get the design patterns right early on by building the vertical slice. It will give you experience with what does work and what does not work with Java/J2EE. Once you are happy with the initial vertical slice then you can apply it across the application. The initial vertical slice should be based on a typical business use case. Refer Q136 in Enterprise section.

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Ensure the system is configurable through property files, xml descriptor files, annotations etc. This will improve flexibility and maintainability. Avoid hard coding any values. Use a constant class for values, which rarely change and use property files, xml descriptor files, annotations etc for values, which can change more frequently (e.g. process flow steps etc) and/or environment related configurations(e.g. server name, server port, LDAP server location etc).

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Design considerations during design, development and deployment phases: designing a fast, secured, reliable, robust, reusable and flexible system require considerations in the following key areas: ™

Performance issues (network overheads, quality of the code etc): Can I make a single coarse-grained network call to my remote object instead of 3 fine-grained calls?

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Concurrency issues (multi-threading etc): What if two threads access my object simultaneously will it corrupt the state of my object?

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Transactional issues (ACID properties): What if two clients access the same data simultaneously? What if one part of the transaction fails, do we rollback the whole transaction? What if the client resubmits the same transactional page again?

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Security issues: Are there any potential security holes for SQL injection or URL injection by hackers?

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Memory issues: Is there any potential memory leak problems? Have we allocated enough heap size for rd the JVM? Have we got enough perm space allocated since we are using 3 party libraries, which generate classes dynamically? (e.g. JAXB, XSLT, JasperReports etc)

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Scalability issues: Will this application scale vertically and horizontally if the load increases? Should this object be serializable? Does this object get stored in the HttpSession?

How would you go about…?

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Maintainability, reuse, extensibility etc: How can we make the software reusable, maintainable and extensible? What design patterns can we use? How often do we have to refactor our code?

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Logging and auditing if something goes wrong can we look at the logs to determine the root cause of the problem?

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Object life cycles: Can the objects within the server be created, destroyed, activated or passivated depending on the memory usage on the server? (e.g. EJB).

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Resource pooling: Creating and destroying valuable resources like database connections, threads etc can be expensive. So if a client is not using a resource can it be returned to a pool to be reused when other clients connect? What is the optimum pool size?

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Caching can we save network trips by storing the data in the server’s memory? How often do we have to clear the cache to prevent the in memory data from becoming stale?

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Load balancing: Can we redirect the users to a server with the lightest load if the other server is overloaded?

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Transparent fail over: If one server crashes can the clients be routed to another server without any interruptions?

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Clustering: What if the server maintains a state when it crashes? Is this state replicated across the other servers?

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Back-end integration: How do we connect to the databases and/or legacy systems?

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Clean shutdown: Can we shut down the server without affecting the clients who are currently using the system?

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Systems management: In the event of a catastrophic system failure who is monitoring the system? Any alerts or alarms? Should we use JMX? Should we use any performance monitoring tools like Tivoli etc?

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Dynamic redeployment: How do we perform the software deployment while the site is running? (Mainly for mission critical applications 24hrs X 7days).

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Portability issues: Can I port this application to a different server 2 years from now?

Q 03: How would you go about identifying performance and/or memory issues in your Java/J2EE application? A 03: Profiling can be used to identify any performance issues or memory leaks. Profiling can identify what lines of code the program is spending the most time in? What call or invocation paths are used to reach at these lines? What kinds of objects are sitting in the heap? Where is the memory leak? Etc. ƒ

There are many tools available for the optimization of Java code like JProfiler, Borland OptimizeIt etc. These tools are very powerful and easy to use. They also produce various reports with graphs. Optimizeit™ Request Analyzer provides advanced profiling techniques that allow developers to analyse the performance behaviour of code across J2EE application tiers. Developers can efficiently prioritize the performance of Web requests, JDBC, JMS, JNDI, JSP, RMI, and EJB so that trouble spots can be proactively isolated earlier in the development lifecycle. Thread Debugger tools can be used to identify threading issues like thread starvation and contention issues that can lead to system crash. Code coverage tools can assist developers with identifying and removing any dead code from the applications.

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Hprof which comes with JDK for free. Simple tool. Java –Xprof myClass java -Xrunhprof:[help]|[=] java -Xrunhprof:cpu=samples, depth=6, heap=sites

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Use operating system process monitors like NT/XP Task Manager on PCs and commands like ps, iostat, netstat, vmstat, uptime, nfsstat etc on UNIX machines.

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Write your own wrapper MemoryLogger and/or PerformanceLogger utility classes with the help of totalMemory() and freeMemory() methods in the Java Runtime class for memory usage and System.currentTimeMillis() method for performance. You can place these MemoryLogger and PerformanceLogger calls strategically in your code. Even better approach than utility classes is using Aspect Oriented Programming (AOP) for pre and post memory and/or performance recording where you have the control of activating memory/performance measurement only when needed. (Refer Q3 – Q5 in Emerging Technologies/Frameworks section).

Q 04: How would you go about minimising memory leaks in your Java/J2EE application? A 04: Java’s’ memory management (i.e. Garbage Collection) prevents lost references and dangling references but it is still possible to create memory leaks in other ways. If the application runs with memory leaks for a long duration you will get the error java.lang.OutOfMemoryError. In Java, typically the memory leak occurs when an object of a longer lifecycle has a reference to the objects of a short life cycle. This prevents the objects with short life cycle being garbage collected. The developer must remember to remove the reference to the short-lived objects from the long-lived objects. Objects with the same life cycle do not cause any problem because the garbage collector is smart enough to deal with the circular references (Refer Q33 in Java section). ƒ

Java collection classes like Hashtable, ArrayList etc maintain references to other objects. So having a long life cycle ArrayList pointing to many short-life cycle objects can cause memory leaks.

ƒ

Commonly used singleton design pattern (Refer Q45 in Java section) can cause memory leaks. Singletons typically have a long life cycle. If a singleton has an ArrayList or a Hashtable then there is a potential for memory leaks.

ƒ

Java programming language includes a finalize method that allows an object to free system resources, in other words, to clean up after itself. However using finalize doesn't guarantee that a class will clean up resources expediently. A better approach for cleaning up resources involves the finally method and an explicit close statement. So freeing up the valuable resource in the finalize method or try {} block instead of finally {} block can cause memory leaks (Refer Q45 in Enterprise section).

Q 05: How would you go about improving performance in your Java/J2EE application? A 05: The performance bottlenecks can be attributed to one or more of the following: Performance optimization considerations

Application level

Java infrastructure

System level

¾ ¾ ¾ ¾ ¾

Application design. Application Server tuning. Application coding. Drivers etc. Database partitioning, tuning etc

¾ ¾

JVM selection JVM tuning (min & max heap size, perm size etc

¾ ¾ ¾ ¾

Network I/O, Disk I/O etc Operating System System configuration, topology etc Hardware (CPU, memory, I/O etc)

Let us discuss some of the aspects in detail: ƒ

Java/J2EE application code related performance bottlenecks: ™

Refer Q63 in Java section.

How would you go about…?

158 ™ ƒ

ƒ

ƒ

Refer Q123, Q125 in Enterprise section.

Java/J2EE design related performance bottlenecks. Application design is one of the most important considerations for performance. A well-designed application will not only avoid many performance pitfalls but will also be easier to maintain and modify during the performance-testing phase of the project. ™

Use proper design patterns to minimise network trips (session facade, value object Refer etc Q83Q87 in Enterprise section).

™

Minimise serialization cost by implementing session beans with remote interfaces and entity beans with local interfaces (applicable to EJB 2.x) or even the session beans can be implemented with local interfaces sharing the same JVM with the Web tier components. For EJB1.x some EJB containers can be configured to use pass-by-reference instead of pass-by-value (pass-by-value requires serialization) Refer Q69, Q82 in Enterprise section.

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Use of multi-threading from a thread-pool (say 10 – 50 threads). Using a large number of threads adversely affects performance by consuming memory through thread stacks and CPU by context switching.

Database related performance bottlenecks. ™

Use proper database indexes. Numeric indices are more efficient than character based indices. Minimise the number of columns in your composite keys. Performing a number of “INSERT” operations is more efficient when fewer columns are indexed and “SELECT” operations are more efficient when, adequately indexed based on columns frequently used in your “WHERE” clause. So it is a trade-off between “SELECT” and “INSERT” operations.

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Minimise use of composite keys or use fewer columns in your composite keys.

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Partition the database for performance based on the most frequently accessed data and least frequently accessed data.

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Identify and optimise your SQL queries causing performance problems (Refer Q97 in Enterprise section).

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De-normalise your tables where necessary for performance (Refer Q98 in Enterprise section).

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Close database connections in your Java code in the finally block to avoid any ‘open cursors’ problem (Refer Q45 in Enterprise section).

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Use optimistic concurrency as opposed to pessimistic concurrency where appropriate (Refer Q78 in Enterprise section).

Application Server, JVM, Operating System, and/or hardware related performance bottlenecks. ™

Application Server: Configure the application server for optimum performance (Refer Q88, Q123 in Enterprise section).

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Operating System: Check for any other processes clogging up the system resources, maximum number of processes it can support or connect, optimise operating system etc.

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Hardware: Insufficient memory, insufficient CPU, insufficient I/O, limitation of hardware configurations, network constraints like bandwidth, message rates etc.

Q 06: How would you go about identifying any potential thread-safety issues in your Java/J2EE application? A 06: When you are writing graphical programs like Swing or Internet programs using servlets or JSPs multi-threading is a necessity for all but some special and/or trivial programs. An application program or a process can have multiple threads like multiple processes that can run on one computer. The multiple threads appear to be doing their work in parallel. When implemented on a multi-processor machine, they can actually work in parallel.

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Unlike processes, threads share the same address space (Refer Q36 in Java section) which means they can read and write the same variables and data structures. So care should be taken to avoid one thread disturbing the work of another thread. Let us look at some of the common situations where care should be taken: ™

Swing components can only be accessed by one thread at a time. A few operations are guaranteed to be thread safe but the most others are not. Generally the Swing components should be accessed through an event-dispatching thread. (Refer Q53 in Java section).

™

A typical Servlet life cycle creates a single instance of each servlet and creates multiple threads to handle the service() method. The multi-threading aids efficiency but the servlet code must be coded in a thread safe manner. The shared resources (e.g. instance variable) should be appropriately synchronized or should only use variables in a read-only manner. (Refer Q16 in Enterprise section).

™

The declaration of variables in JSP is not thread-safe, because the declared variables end up in the generated servlet as an instance variable, not within the body of the _jspservice() method. (Refer Q34 in Enterprise section).

™

Struts framework action classes are not thread-safe. (Refer Q113 in Enterprise section).

™

Some Java collection classes like Hashmap, ArrayList etc are not thread-safe. (Refer Q13 in Java section).

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Some of the Java core library classes are not thread safe. For e.g. java.util.SimpleDateFormat, java.util.Locale etc.

Q 07: How would you go about identifying any potential transactional issues in your Java/J2EE application? A 07: ™

When a connection is created, it is in auto-commit mode. This means that each individual SQL statement is treated as a transaction and will be automatically committed immediately after it is executed. The way to allow two or more statements to be grouped into a transaction is to disable auto-commit mode. (Refer Q43 in Enterprise section). Disabling auto-commit mode can improve performance by minimising number of times it accesses the database.

™

A transaction is often described by ACID properties (Atomic, Consistent, Isolated and Durable). A distributed transaction is an ACID transaction between two or more independent transactional resources like two separate databases. For a transaction to commit successfully, all of the individual resources must commit successfully. If any of them are unsuccessful, the transaction must rollback in all of the resources. A 2-phase commit is an approach for committing a distributed transaction in 2 phases. Refer Q73 in Enterprise section.

™

Isolation levels provide a degree of control of the effects one transaction can have on another concurrent transaction. Concurrent effects are determined by the precise ways in which, a particular relational database handles locks and its drivers may handle these locks differently. Isolation levels are used to overcome transactional problems like lost update, uncommitted data (aka dirty reads), inconsistent data (aka. phantom update), and phantom insert. Higher isolation levels can adversely affect performance at the expense of data accuracy. Refer Q72 in Enterprise section. Isolation Level Read Uncommitted Read Committed Repeatable Read Serializable

Lost Update Prevented by DBMS Prevented by DBMS Prevented by DBMS Prevented by DBMS

Uncommitted Data Can happen Prevented by DBMS Prevented by DBMS Prevented by DBMS

Inconsistent Data Can happen Can happen Prevented by DBMS Prevented by DBMS

Phantom Insert Can happen Can happen Can happen Prevented by DBMS

™

Decide between optimistic and pessimistic concurrency control. (Refer Q78 in Enterprise section).

™

Evaluate a strategy to determine if the data is stale when using strategies to cache data. (Refer Q79 in Enterprise section).

EJB related transactional issues: ™

Set the appropriate transactional attributes for the EJBs. (Refer Q71 in Enterprise section).

™

Set the appropriate isolation level for the EJB. The isolation level should not be any more restrictive than it has to be. Higher isolation levels can adversely affect performance. (Refer Q72 in Enterprise section). Isolation levels are application server specific and not part of the standard EJB configuration.

How would you go about…?

160 ™

In EJB 2.x, transactions are rolled back by the container when a system exception is thrown. When an application exception is thrown then the transactions are not rolled back by the container. So the developer has to roll it back using ctx.setRollbackOnly() call. (Refer Q76, Q77 in Enterprise section).

™

Detect doomed transactions to avoid performing any unnecessary compute intensive operations. (Refer Q72 in Enterprise section).

Q 08: How would you go about applying the Object Oriented (OO) design concepts in your Java/J2EE application? A 08: Question

Answer

What are the key characteristics of an OO language like Java?

A true object oriented language should support the following 3 characteristics: ™

Encapsulation (aka information hiding): implements information hiding and modularity (abstraction).

™

Polymorphism: The same message sent to different objects, results in behaviour that is dependent on the nature of the object receiving the message.

™

Inheritance: Encourages code reuse and code organisation by defining the new class based on the existing class.

What is dynamic binding? Dynamic binding (aka late binding): The dynamic binding is used to implement polymorphism. Objects could come from local process or from across the network from a remote process. We should be able to send messages to objects without having to know their types at the time of writing the code. Dynamic binding provides maximum flexibility at the execution time. Usually dynamic binding or late binding takes a small performance hit. Refer Q8 in Java section. Let us take an example to illustrate dynamic binding through polymorphic behaviour: Say you have a method in Java void draw(Shape s) { s.erase(); // ... s.draw(); } The above method will talk to any shape, so it is independent of the specific type of object it is erasing and drawing. Now let us look at some other program, which is making use of this draw(Shape s) method: Circle cir = new Circle(); Square sq = new Square(); draw(cir); draw(sq); So the interesting thing is that the method call to draw(Shape s) will cause different code to be executed. So you send a message to an object even though you don’t know what specific type it is and the right thing happens. This is called dynamic binding, which gives you polymorphic behaviour. How will you decide whether to use an interface or an abstract class?

™

Abstract Class: Often in a design, you want the base class to present only an interface for its derived classes. That is, you don’t want anyone to actually create an object of the base class, only to upcast to it so that its interface can be used. This is accomplished by making that class abstract using the abstract key word. If anyone tries to make an object of an abstract class, the compiler prevents them. This is a tool to enforce a particular design.

™

Interface: The interface key word takes the concept of an abstract class one step further by preventing any function definitions at all. An interface is a very useful and commonly used tool, as it provides the perfect separation of interface and implementation. In addition, you can combine many interfaces together, if you wish. (You cannot inherit from more than one regular class or abstract class.)

Now the design decision…

How would you go about…?

161

When to use an Abstract Class: Abstract classes are excellent candidates inside of application frameworks. Abstract classes let you define some default behaviour and force subclasses to provide any specific behaviour. When to use an Interface: If you need to change your design frequently, I prefer using interface to abstract. For example, the strategy pattern lets you swap new algorithms and processes into your program without altering the objects that use them. Example: Strategy Design Pattern. Another justification of interfaces is that they solved the ‘diamond problem’ of traditional multiple inheritance. Java does not support multiple inheritances. Java only supports multiple interface inheritance. Interface will solve all the ambiguities caused by this ‘diamond problem’. Refer Q10 in Java section. Interface inheritance vs. Implementation inheritance: Prefer interface inheritance to implementation inheritance because it promotes the design concept of coding to an interface and reduces coupling. Interface inheritance can achieve code reuse with the help of object composition. Refer Q08 in Java section. Why abstraction is important in Object Oriented programming?

The software you develop should optimally cater for the current requirements and problems and also should be flexible enough to easily handle future changes. Abstraction is an important OO concept. The ability for a program to ignore some aspects of the information that it is manipulating, ie. Ability to focus on the essential. Each object in the system serves as a model of an abstract "actor" that can perform work, report on and change its state, and "communicate" with other objects in the system, without revealing how these features are implemented. Abstraction is the process where ideas are distanced from the concrete implementation of the objects. The concrete implementation will change but the abstract layer will remain the same. Let us look at an analogy: When you drive your car you do not have to be concerned with the exact internal working of your car (unless you are a mechanic). What you are concerned with is interacting with your car via its interfaces like steering wheel, brake pedal, accelerator pedal etc. Over the years a car’s engine has improved a lot but its basic interface has not changed (ie you still use steering wheel, brake pedal, accelerator pedal etc to interact with your car). This means that the implementation has changed over the years but the interface remains the same. Hence the knowledge you have of your car is abstract.

Explain black-box reuse and whitebox reuse? Should you favour Inheritance (whitebox reuse) or aggregation (black-box reuse)?

Black-box reuse is when a class uses another class without knowing the internal contents of it. The black-box reuses are: ™ ™ ™ ™

Dependency is the weakest type of black-box reuse. Association is when one object knows about or has a relationship with the other objects. Aggregation is the whole part relationship where one object contains one or more of the other objects. Composition is a stronger whole part relationship

Refer Q107, Q108 in Enterprise section White-box reuse is when a class knows internal contents of another class. E.g. inheritance is used to modify implementation for reusability. Aggregation (Black-box reuse)

Inheritance (White-box reuse)

Defined dynamically or at run time via object references. Since only interfaces are used, it has the advantage of maintaining the integrity (ie encapsulation).

Inheritance is defined statically or at compile time. Inheritance allows an easy way to modify implementation for reusability.

Disadvantage of aggregation is that it increases the number of objects and relationships.

A disadvantage of inheritance is that it breaks encapsulation, which implies implementation dependency. This means when you want to carry out the redesign the super class (ie parent class) has to be modified or replaced which is more likely to affect the subclasses as well. In general it will affect the whole inheritance hierarchy. Verdict: So the tendency is to favour aggregation over inheritance.

How would you go about…?

162 What is your understanding on Aspect Oriented Programming (AOP)?

Aspect-Oriented Programming (AOP) complements OO programming by allowing developers to dynamically modify the static OO model to create a system that can grow to meet new requirements. AOP allows us to dynamically modify our static model to include the code required to fulfil the secondary requirements (like auditing, logging, security, exception handling etc) without having to modify the original static model (in fact, we don't even need to have the original code). Better still, we can often keep this additional code in a single location rather than having to scatter it across the existing model, as we would have to if we were using OO on its own. (Refer Q3 –Q5 in Emerging Technologies/Frameworks section.) For example A typical Web application will require a servlet to bind the HTTP request to an object and then passes to the business handler object to be processed and finally return the response back to the user. So initially only a minimum amount of code is required. But once you start adding all the other additional secondary requirements (aka crosscutting concerns) like logging, auditing, security, exception-handling etc the code will inflate to 2-4 times its original size. This is where AOP can help.

Q 09: How would you go about applying the UML diagrams in your Java/J2EE project? A 09: Question

Answer

Explain the key relationships in the use case diagrams?

Refer Q107 in Enterprise section. Use case has 4 types of relationships: Between actor and use case ™

Association: Between actor and use case. May be navigable in both directions according to the initiator of the communication between the actor and the usecase.

Between use cases ™

Extends: This is an optional extended behaviour of a use case. This behaviour is executed only under certain conditions such as performing a security check etc.

™

Includes: This specifies that the base use case needs an additional use case to fully describe its process. It is mainly used to show common functionality that is shared by several use cases.

™

Inheritance (or generalization): Child use case inherits the behaviour of its parent. The child may override or add to the behaviour of the parent. U s e c a s e d ia g r a m

a s s o c ia tio n *

*

E n r o ll in U n iv e r s i ty

R e g is tr a r

< < in c lu d e > > E n r o ll in s e m in a r

a s s o c ia tio n

< < e x te n d > > S tu d e n t p e r f o r m s e c u r ity check

in h e r ita n c e

E n r o ll fa m ily m e m b e rs

in h e r it a n c e

a s s o c ia t io n

I n t e r n a tio n a l S tu d e n t

What is the main difference between the collaboration diagram and the sequence diagram? When to use various UML diagrams?

N o te : < < e x t e n d > > r e la tio n s h ip is c o n d itio n a l. Y o u d o n o t k n o w if o r w h e n e x te n d in g u s e c a s e w ill b e in v o k e d . < < in c lu d e > > r e la t io n s h ip is s im ila r to a p r o c e d u r e c a ll. in h e r it a n c e : e x te n d s th e b e h a v io r o f th e p a re n t u s e c a s e o r a c to r.

Refer Q107 in Enterprise section: Collaboration diagrams convey the same message as sequence diagrams but the collaboration diagrams focus on object roles instead of times in which the messages are sent. The sequence diagram is time line driven. Refer Q107 in Enterprise section. ™

Use case diagrams: ƒ ƒ ƒ

Determining the user requirements. New use cases often generate new requirements. Communicating with clients. The simplicity of the diagram makes use case diagrams a good way for designers and developers to communicate with clients. Generating test cases. Each scenario for the use case may suggest a suite of test

How would you go about…?

163

cases. ™

Class diagrams: ƒ ƒ

™

Interaction diagrams (Sequence and/or Collaboration diagrams): ƒ

™

™

Class diagrams are the backbone of Object Oriented methods. So they are used frequently. Class diagrams can have a conceptual perspective and an implementation perspective. During the analysis draw the conceptual model and during implementation draw the implementation model.

When you want to look at behaviour of several objects within a single use case. If you want to look at a single object across multiple use cases then use statechart diagram as described below.

State chart diagrams: ƒ

Statechart diagrams are good at describing the behaviour of an object across several use cases. But they are not good at describing the interaction or collaboration between many objects. Use interaction and/or activity diagrams in conjunction with the statechart diagram to communicate complex operations involving multi-threaded programs etc.

ƒ

Use it only for classes that have complex state changes and behaviour. For example: the User Interface (UI) control objects, Objects shared by multi-threaded programs etc.

Activity diagram: ƒ

Activity and Statechart diagrams are generally useful to express complex operations. The great strength of activity diagrams is that they support and encourage parallel behaviour. An activity and statechart diagrams are beneficial for workflow modelling with multi threaded programming.

Q 10: How would you go about describing the software development processes you are familiar with? A 10: In addition to technical questions one should also have a good understanding of the software development process. Question What is the key difference between the waterfall approach and the iterative approach to software development? How to decide which one to use?

Answer Refer Q103 – Q105 in Enterprise section Waterfall approach is sequential in nature. The iterative approach is non-sequential and incremental. The iterative and incremental approach has been developed based on the following: •

You can't express all your needs up front. It is usually not feasible to define in detail (that is, before starting full-scale development) the operational capabilities and functional characteristics of the entire system. These usually evolve over time as development progresses.

•

Technology changes over time. Some development lifecycle spans a long period of time during which, given the pace at which technology evolves, significant technological shifts may occur.

•

Complex systems. This means it is difficult to cope with them adequately unless you have an approach for mastering complexity.

How to decide which one to use? Waterfall approach is more suitable in the following circumstances: •

• •

Have a small number of unknowns and risks. That is if • It has a known domain. • The team is experienced in current process and technology. • There is no new technology. • There is a pre-existing architecture baseline. Is of short duration (two to three months). Is an evolution of an existing system?

The iterative approach is more suitable (Refer Q136 in Enterprise Section) •

Have a large number of unknowns and risks. So it pays to design, develop and test a vertical slice iteratively and then replicate it through other iterations. That is if

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• • • • •

Have you used extreme programming techniques? Explain?

Integrating with new systems. New technology and/or architecture. The team is fairly keen to adapt to this new process.

Is of large duration (longer than 3 months). Is a new system?

Extreme Programming (or XP) is a set of values, principles and practices for rapidly developing highquality software that provides the highest value for the customer in the fastest way possible. XP is a minimal instance of RUP. XP is extreme in the sense that it takes 12 well-known software development "best practices" to their logical extremes. The 12 core practices of XP are: 1.

The Planning Game: Business and development cooperate to produce the maximum business value as rapidly as possible. The planning game happens at various scales, but the basic rules are always the same: ƒ

Business comes up with a list of desired features for the system. Each feature is written out as a user story (or PowerPoint screen shots with changes highlighted), which gives the feature a name, and describes in broad strokes what is required. User stories are typically written on 4x6 cards.

ƒ

Development team estimates how much effort each story will take, and how much effort the team can produce in a given time interval (i.e. the iteration).

ƒ

Business then decides which stories to implement in what order, as well as when and how often to produce production releases of the system.

2.

Small releases: Start with the smallest useful feature set. Release early and often, adding a few features each time.

3.

System metaphor: Each project has an organising metaphor, which provides an easy to remember naming convention.

4.

Simple design: Always use the simplest possible design that gets the job done. The requirements will change tomorrow, so only do what's needed to meet today's requirements.

5.

Continuous testing: Before programmers add a feature, they write a test for it. Tests in XP come in two basic flavours. ƒ

Unit tests are automated tests written by the developers to test functionality as they write it. Each unit test typically tests only a single class, or a small cluster of classes. Unit tests are typically written using a unit-testing framework, such as JUnit.

ƒ

Customer to test that the overall system is functioning as specified, defines acceptance tests (aka Functional tests). Acceptance tests typically test the entire system, or some large chunk of it. When all the acceptance tests pass for a given user story, that story is considered complete. At the very least, an acceptance test could consist of a script of user interface actions and expected results that a human can run. Ideally acceptance tests should be automated using frameworks like Canoo Web test, Selenium Web test etc.

6.

Refactoring: Refactor out any duplicate code generated in a coding session. You can do this with confidence that you didn't break anything because you have the tests.

7.

Pair Programming: All production code is written by two programmers sitting at one machine. Essentially, all code is reviewed as it is written.

8.

Collective code ownership: No single person "owns" a module. Any developer is expected to be able to work on any part of codebase at any time.

9.

Continuous integration: All changes are integrated into codebase at least daily. The tests have to run 100% both before and after integration. You can use tools like Ant, CruiseControl, and Maven etc to continuously build and integrate your code.

10.

40-Hour Workweek: Programmers go home on time. In crunch mode, up to one week of overtime is allowed. But multiple consecutive weeks of overtime are treated as a sign that something is very wrong with the process.

11.

On-site customer: Development team has continuous access to a real live customer or business owner, that is, someone who will actually be using the system. For commercial software with lots of customers, a customer proxy (usually the product manager, Business

How would you go about…?

165

Analyst etc) is used instead. 12.

Coding standards: Everyone codes to the same standards. Ideally, you shouldn't be able to tell by looking at it, which developer on the team has touched a specific piece of code.

A typical extreme programming project will have: • • • • •

Have you used agile (i.e. Lightweight) software development methodologies?

All the programmers in a room together usually sitting around a large table. Fixed number of iterations where each iteration takes 1-3 weeks. At the beginning of each iteration get together with the customer. Pair-programming. Writing test cases first (i.e. TDD). Delivery of a functional system at the end of 1-3 week iteration.

Agile (i.e. lightweight) software development process is gaining popularity and momentum across organizations. Several methodologies fit under this agile development methodology banner. All these methodologies share many characteristics like iterative and incremental development, test driven development (i.e. TDD), stand up meetings to improve communication, automatic testing, build and continuous integration of code etc. Among all the agile methodologies XP is the one which has got the most attention. Different companies use different flavours of agile methodologies by using different combinations of methodologies (e.g. primarily XP with other methodologies like Scrum, FDD, TDD etc). Refer Q136 in Enterprise section.

Q 11: How would you go about applying the design patterns in your Java/J2EE application? A 11: It is really worth reading books and articles on design patterns. It is sometimes hard to remember the design patterns, which you do not use regularly. So if you do not know a particular design pattern you can always honestly say that you have not used it and subsequently suggest that you can explain another design pattern, which you have used recently or more often. It is always challenging to decide, which design pattern to use when? How do you improve your design pattern skills? Practice, practice, practice. I have listed some of the design patterns below with scenarios and examples: Note: To keep it simple, System.out.println(…) is used. In real practice, use logging frameworks like log4j. Also package constructs are not shown. In real practice, each class should be stored in their relevant packages like com.items etc. Feel free to try these code samples by typing them into a Java editor of your choice and run the main class Shopping. Also constants should be declared in a typesafe manner as shown below: /** * use typesafe enum pattern as shown below if you are using below JDK 1.5 or use “enum” if you are using JDK 1.5 */ public class ItemType { private final String name; public static final ItemType Book = new ItemType("book"); public static final ItemType CD = new ItemType("cd"); public static final ItemType COSMETICS = new ItemType("cosmetics"); public static final ItemType CD_IMPORTED = new ItemType("cd_imported"); private ItemType(String name) {this.name = name;} public String toString() {return name;} //add compareTo(), readResolve() methods etc as required ... }

Scenario: A company named XYZ Retail is in the business of selling Books, CDs and Cosmetics. Books are sales tax exempt and CDs and Cosmetics have a sales tax of 10%. CDs can be imported and attracts an import tax of 5%. Write a shopping basket program, which will calculate extended price (qty * (unitprice + tax)) inclusive of tax for each item in the basket, total taxes and grand total. Solution: Sample code for the items (i.e. Goods) sold by XYZ Retail. Let’s define an Item interface to follow the design principle of code to an interface not to an implementation. CO public interface Item { public static final int TYPE_BOOK = 1; public static final int TYPE_CD = 2; public static final int TYPE_COSMETICS = 3; public static final int TYPE_CD_IMPORTED = 4; public double getExtendedTax();

166

How would you go about…?

public double getExtendedTaxPrice() throws ItemException; public void setImported(boolean b); public String getDescription(); }

The following class Goods cannot be instantiated (since it is abstract). You use this abstract class to achieve code reuse. /** * abstract parent class, which promotes code reuse for all the subclasses code reuse is achieved through implementation inheritance. * like Book, CD, andCosmetics. implements interface Item to * promote design principle code to interface not to an implementation. */ public abstract class Goods implements Item { //define attributes 11 «interface» private String description; Goods Tax Item private int qty; -qty : int -salesTax : double private double price; -price : double -importTax : double private Tax tax = new Tax(); -tax : Tax +getExtendedTax() : double +getExtendedTaxPrice() : double +isTaxed() : boolean +isImported() : boolean

public Goods(String description, int qty, double price) { this.description = description; this.qty = qty; this.price = price; } protected abstract boolean isTaxed(); protected abstract boolean isImported(); public double getExtendedTax() { tax.calculate(isTaxed(), isImported(), price); return this.tax.getTotalUnitTax() * qty; }

CD

Book

Cosmetics

-isTaxed : boolean -isImported : boolean

-isTaxed : boolean -isImported : boolean

-isTaxed : boolean -isImported : boolean

public double getExtendedTaxPrice() throws ItemException { if (tax == null) { throw new ItemException("Tax should be calculated first:"); } return qty * (this.tax.getTotalUnitTax() + price); } //getters and setters go here for attributes like description etc public String getDescription() { return description; } public String toString() { return qty + " " + description + " : "; } }

The Book, CD and Cosmetics classes can be written as shown below: public class Book extends Goods { private boolean isTaxed = false; private boolean isImported = false; public Book(String description, int qty, double price) { super(description, qty, price); } public boolean isTaxed() { return isTaxed; } public boolean isImported() { return isImported; } public void setImported(boolean b) { isImported = b; } }

How would you go about…?

167

public class CD extends Goods { private boolean isTaxed = true; private boolean isImported = false; public CD(String description, int qty, double price) { super(description, qty, price); } public boolean isTaxed() { return isTaxed; } public boolean isImported() { return isImported; } public void setImported(boolean b) { isImported = b; } } public class Cosmetics extends Goods { private boolean isTaxed = true; private boolean isImported = false; public Cosmetics(String description, int qty, double price) { super(description, qty, price); } public boolean isTaxed() { return isTaxed; } public boolean isImported() { return isImported; } public void setImported(boolean b) { isImported = b; } }

Alternative solution: Alternatively, instead of using inheritance, we can use object composition to achieve code reuse as discussed in Q8 in Java section. If you were to use object composition instead of inheritance, you would have classes Book, CD and Cosmetics implementing the Item interface directly (Goods class would not be required), and make use of a GoodsHelper class to achieve code reuse through composition. interface inheritance where code reuse is achieved through composition [GoodsHelper]. code not shown.

GoodsHelper 1 1 CD

1 1 Book

1 Cosmetics

-isTaxed : boolean -isTaxed : boolean -isTaxed : boolean -isImported : boolean -isImported : boolean -isImported : boolean -helper : GoodsHelper -helper : GoodsHelper -helper : GoodsHelper

«interface» Item

Let’s define a Tax class, which is responsible for calculating the tax. The Tax class is composed in your Goods class, which makes use of object composition to achieve code reuse. public class Tax {

168

How would you go about…?

//stay away from hard coding values. Define constants or read from a “.properties” file public static final double SALES_TAX = 0.10; //10% public static final double IMPORT_TAX = 0.05; //5% private double salesTax = 0.0; private double importTax = 0.0; public void calculate(boolean isTaxable, boolean isImported, double price) { if (isTaxable) { salesTax = price * SALES_TAX; } if (isImported) { importTax = price * IMPORT_TAX; } } public double getTotalUnitTax() { return this.salesTax + this.importTax; } }

Factory method pattern: To create the items shown above we could use the factory method pattern as described in Q46 in Java section. We would also implement the factory class as a singleton using the singleton design pattern as described in Q45 in Java section. The factory method design pattern instantiates a class in a more flexible way than directly calling the constructor. It loosely couples your calling code from the Items it creates like CD, Book, etc. Let’s look at why factory method pattern is more flexible: ƒ

Sometimes factory methods have to return a single instance of a class instead of creating new objects each time or return an instance from a pool of objects.

ƒ

Factory methods have to return a subtype of the type requested. It also can request the caller to refer to the returned object by its interface rather than by its implementation, which enables objects to be created without making their implementation classes public.

ƒ

Sometimes old ways of creating objects can be replaced by new ways of creating the same objects or new classes can be added using polymorphism without changing any of the existing code which uses these objects. For example: Say you have a Fruit abstract class with Mango and Orange as its concrete subclasses, later on you can add an Apple subclass without breaking the code which uses these objects.

The factory method patterns consist of a product class hierarchy and a creator class hierarchy. /** * ItemFactory is responsible for creating Item objects like CD, Book, and Cosmetics etc */ public abstract class ItemFactory { public abstract Item getItem(int itemType, String description, int qty, double price) throws ItemException; } /** * GoodsFactory responsible for creating Item objects like CD, Book, and Cosmetics etc */ public class GoodsFactory extends ItemFactory { protected GoodsFactory() { } //protected so that only ItemFactorySelector within this package can instantiate it to provide a single // point of access (i.e singleton). /** * Factory method, which decides how to create Items. * * Benefits are: -- loosely-couples the client (i.e ShoppingBasketBuilder class) from Items such as CD, Book, and Cosmetics etc. * In future if we need to create a Book item, which is imported, we can easily incorporate this by adding a new * item.TYPE_BOOK_IMPORTED and subsequently adding following piece of code as shown: * * else if(itemType == TYPE_BOOK_IMPORTED){ * item = new Book(description, qty,price); * item.setIsImported(true); *} * * --It is also possible to create an object cache or object pool of our items instead of creating a new instance

How would you go about…?

169

* every time without making any changes to the calling class. * * --Java does not support overloaded constructors which take same parameter list. Instead, use several factory methods. * E.g. getImportedItem(int itemType, String description, int qty, double price). */ public Item getItem(int itemType, String description, int qty, double price) throws ItemException { Item item = null; if (itemType == Item.TYPE_BOOK) { «interface» item = new Book(description, qty, price); ItemFactory Goods Item } else if (itemType == Item.TYPE_CD) { item = new CD(description, qty, price); +getItem() } else if (itemType == Item.TYPE_CD_IMPORTED) { item = new CD(description, qty, price); item.setImported(true); } else if (itemType == Item.TYPE_COSMETICS) { item = new Cosmetics(description, qty, price); GoodsFactory CD Book Cosmetics } else { throw new ItemException("Invalid ItemType=" + itemType); } +getItem() return item; } }

Let’s use the abstract factory pattern to create an ItemFactory and the singleton pattern to provide a single point of access to the ItemFactory returned. Abstract factory pattern: This pattern is one level of abstraction higher than the factory method pattern because you have an abstract factory (or factory interface) and have multiple concrete factories. Abstract factory pattern usually has a specific method for each concrete type being returned (e.g. createCircle(), createSquare() etc). Alternatively you can have a single method e.g. createShape(…). Singleton pattern: Ensures that a class has only one instance and provides a global point of access to it (Refer Q45 in Java section). E.g. a DataSource should have only a single instance where it will supply multiple connections from its single DataSource pool. /** * Abstract factory class which creates a singleton ItemFactory dynamically based on factory name supplied. * Benefits of singleton: -- single instance of the ItemFactory -- single point of access (global access within the JVM and the class loader) */ public class ItemFactorySelector { private static ItemFactory objectFactorySingleInstance = null; private static final String FACTORY_NAME = "com.item.GoodsFactory"; public static ItemFactory getItemFactory() { try { if (objectFactorySingleInstance == null) { //Dynamically instantiate factory and factory name can also be read from a properties file. //in future if we need a CachedGoodsFactory which caches Items to improve memory usage then //we can modify the FACTORY_NAME to "com.item.CachedGoodsFactory" or conditionally select one of many factories. Class klassFactory = Class.forName(FACTORY_NAME); objectFactorySingleInstance = (ItemFactory) klassFactory.newInstance(); } } catch (ClassNotFoundException cnf) { throw new RuntimeException("Cannot create the ItemFactory: " + cnf.getMessage()); }catch (IllegalAccessException iae) { throw new RuntimeException("Cannot create the ItemFactory: " + iae.getMessage()); }catch (InstantiationException ie) { throw new RuntimeException("Cannot create the ItemFactory: " + ie.getMessage()); } return objectFactorySingleInstance; } }

Now we should build a more complex shopping basket object step-by-step, which is responsible for building a basket with items like CD, Book etc and calculating total tax for the items in the basket. The builder design pattern is used to define the interface ItemBuilder and the concrete class, which implements this interface, is named ShoppingBasketBuilder. Builder pattern: The subtle difference between the builder pattern and the factory pattern is that in builder pattern, the user is given the choice to create the type of object he/she wants but the construction process is the same. But

170

How would you go about…?

with the factory method pattern the factory decides how to create one of several possible classes based on data provided to it. …//package & import statements public interface ItemBuilder { public void buildBasket(int itemType, String description, int qty, double unit_price) throws ItemException; public double calculateTotalTax() throws ItemException; public double calculateTotal() throws ItemException; public void printExtendedTaxedPrice() throws ItemException; public Iterator getIterator(); } …//package & import statements /** * Builder pattern: To simplify complex object creation by defining a class whose purpose is to build instances of another class. * There is a subtle difference between a builder pattern and the factory pattern is that in builder pattern, the user is given * the choice to create the type of object he/she wants but the construction process is the same. But with the factory method * pattern the factory decides how to create one of several possible classes based on data provided to it. */ public class ShoppingBasketBuilder implements ItemBuilder { private List listItems = null; private void addItem(Item item) { if (listItems == null) { listItems = new ArrayList(20); } listItems.add(item); } /** * builds a shopping basket */ public void buildBasket(int itemType, String description, int qty, double unit_price) throws ItemException { //get the single instance of GoodsFactory using the singleton pattern //no matter how many times you call getInstance() you get access to the same instance. ItemFactory factory = ItemFactorySelector.getItemFactory(); //use factory method pattern to create item objects, based on itemType supplied to it. Item item = factory.getItem(itemType, description, qty, unit_price); this.addItem(item); //adds the item to the basket } /** * calculates total tax on the items in the built basket */ public double calculateTotalTax() throws ItemException { if (listItems == null) { throw new ItemException("No items in the basket"); } double totalTax = 0.0; Iterator it = listItems.iterator(); while (it.hasNext()) { Item item = (Item) it.next(); totalTax += item.getExtendedTax(); } return totalTax; } /** * calculates total price on the items in the built basket */ public double calculateTotal() throws ItemException { if (listItems == null) { throw new ItemException("No items in the basket"); } double total = 0.0; Iterator it = listItems.iterator(); while (it.hasNext()) { Item item = (Item) it.next(); total += item.getExtendedTaxPrice();

ItemBuilder +buildBasket() +calculateTotalTax() +calculateTotal() +printExtendedTax() +getItemIterator()

aProduct : Item

ShoppingBasketBuilder *

-listItems : List +buildBasket() +calculateTotalTax() +calculateTotal() +printExtendedTax() +getItemIterator()

1 «call» ItemFactory +getItem()

GoodsFactory +getItem() +getInstance()

aProduct:Item

How would you go about…?

171

} return total; } /** * prints individual prices of the items in the built basket */ public void printExtendedTaxedPrice() throws ItemException { if (listItems == null) { throw new ItemException("No items in the basket"); } double totalTax = 0.0; Iterator it = listItems.iterator(); while (it.hasNext()) { Item item = (Item) it.next(); System.out.println(item + "" + item.getExtendedTaxPrice()); } } public Iterator getIterator() { return listItems.iterator(); } }

Finally, the calling-code, which makes use of our shopping basket builder pattern to build the shopping basket step-bystep and also calculates the taxes and the grand total for the items in the shopping basket. …//package & import statements public class Shopping { /** * Class with main(String[] args) method which initially gets loaded by the * class loader. Subsequent classes are loaded as they are referenced in the program. */ public static void main(String[] args) throws ItemException { process(); } public static void process() throws ItemException { //------creational patterns: singleton, factory method and builder design patterns---------------System.out.println("----create a shopping basket with items ---"); //Shopping basket using the builder pattern ItemBuilder builder = new ShoppingBasketBuilder(); //build basket of items using a builder pattern builder.buildBasket(Item.TYPE_BOOK, "Book - IT", 1, 12.00); builder.buildBasket(Item.TYPE_CD, "CD - JAZZ", 1, 15.00); builder.buildBasket(Item.TYPE_COSMETICS, "Cosmetics - Lipstick", 1, 1.0); //lets print prices and taxes of this built basket double totalTax = builder.calculateTotalTax(); builder.printExtendedTaxedPrice(); System.out.println("Sales Taxes: " + totalTax); System.out.println("Grand Total: " + builder.calculateTotal()); System.out.println("----- After adding an imported CD to the basket ----"); //Say now customer decides to buy an additional imported CD builder.buildBasket(Item.TYPE_CD_IMPORTED, "CD - JAZZ IMPORTED", 1, 15.00); //lets print prices and taxes of this built basket with imported CD added totalTax = builder.calculateTotalTax(); builder.printExtendedTaxedPrice(); System.out.println("Sales Taxes: " + totalTax); System.out.println("Grand Total: " + builder.calculateTotal()); } }

Running the above code produces an output of: ----create a shopping basket with items --1 Book - IT : 12.0 1 CD - JAZZ : 16.5 1 Cosmetics - Lipstick : 1.1 Sales Taxes: 1.6

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How would you go about…?

Grand Total: 29.6 ----- After adding an imported CD to the basket ---1 Book - IT : 12.0 1 CD - JAZZ : 16.5 1 Cosmetics - Lipstick : 1.1 1 CD - JAZZ IMPORTED : 17.25 Sales Taxes: 3.85 Grand Total: 46.85

Scenario: The XYZ Retail wants to evaluate a strategy to determine items with description longer than 15 characters because it won’t fit in the invoice and items with description starting with “CD” to add piracy warning label. Solution: You can implement evaluating a strategy to determine items with description longer than 15 characters and description starting with “CD” applying the strategy design pattern as shown below: Strategy pattern: The Strategy pattern lets you build software as a loosely coupled collection of interchangeable parts, in contrast to a monolithic, tightly coupled system. Loose coupling makes your software much more extensible, maintainable, and reusable. The main attribute of this pattern is that each strategy encapsulates algorithms i.e. it is not data based but algorithm based. Refer Q10, Q55 in Java section. Example: You can draw borders around almost all Swing components, including panels, buttons, lists, and so on. Swing provides numerous border types for its components: bevel, etched, line, titled, and even compound. JComponent class, which acts as the base class for all Swing components by implementing functionality common to all Swing components, draws borders for Swing components, using strategy pattern. public interface CheckStrategy { public boolean check(String word); } public class LongerThan15 implements CheckStrategy { public static final int LENGTH = 15; //constant public boolean check(String description) { if (description == null) return false; else return description.length() > LENGTH; }

LongerThan15 +check() : boolean

«interface» CheckStrategy +check() : boolean

StartsWithCD +check() : boolean

} public class StartsWithCD implements CheckStrategy { public static final String STARTS_WITH = "cd"; public boolean check(String description) { String s = description.toLowerCase(); if (description == null || description.length() == 0) return false; else return s.startsWith(STARTS_WITH); } }

Scenario: The XYZ retail has decided to count the number of items, which satisfy the above strategies. Solution: You can apply the decorator design pattern around your strategy design pattern. Refer Q20 in Java section for the decorator design pattern used in java.io.*. The decorator acts as a wrapper around the CheckStrategy objects where by call the check(…) method on the CheckStrategy object and if it returns true then increment the counter. The decorator design pattern can be used to provide additional functionality to an object of some kind. The key to a decorator is that a decorator "wraps" the object decorated and looks to a client exactly the same as the object wrapped. This means that the decorator implements the same interface as the object it decorates. Decorator design pattern: You can think of a decorator as a shell around the object decorated. The decorator catches any message that a client sends to the object instead. The decorator may apply some action and then pass the message it received on to the decorated object. That object probably returns a value to the decorator which may again apply an action to that result, finally sending the (perhaps-modified) result to the original client. To the client the decorator is invisible. It just sent a message and got a result. However the decorator had two chances to enhance the result returned.

How would you go about…?

173

public class CountDecorator implements CheckStrategy { LongerThan15

private CheckStrategy cs = null; private int count = 0;

+check() : boolean

public CountDecorator(CheckStrategy cs) { this.cs = cs; } public boolean check(String description) { boolean isFound = cs.check(description); if (isFound) this.count++; return isFound; } public int count() { return this.count; } public void reset() { this.count = 0; }

«interface» CheckStrategy +check() : boolean

*

StartsWithCD +check() : boolean

CountDecorator +check() : boolean

1

There is a subtle difference between the decorator pattern and the proxy pattern is that, the main intent of the decorator pattern is to enhance the functionality of the target object whereas the main intent of the proxy pattern is to control access to the target object.

A decorator object’s interface must conform to the interface of the component it decorates

}

Now, let’s see the calling class Shopping: //…. package & import statements public class Shopping { //... public static void process() throws ItemException { ... Iterator it = builder.getIterator(); boolean bol = false; CheckStrategy strategy = null; it = builder.getIterator(); //for starting with CD strategy = new StartsWithCD(); strategy = new CountDecorator(strategy); while (it.hasNext()) { Item item = (Item) it.next(); bol = strategy.check(item.getDescription()); System.out.println("\n" + item.getDescription() + " --> " + bol); } System.out.println("No of descriptions starts with CD -->" + ((CountDecorator) strategy).count()); it = builder.getIterator(); //for starting with CD strategy = new LongerThan15(); strategy = new CountDecorator(strategy); while (it.hasNext()) { Item item = (Item) it.next(); bol = strategy.check(item.getDescription()); System.out.println("\n" + item.getDescription() + " --> " + bol); } System.out.println("No of descriptions longer than 15 characters -->" + ((CountDecorator) strategy).count()); } }

Running the above code produces an output of: ----count item description starting with 'cd' or longer than 15 characters ---------------------- description satarting with cd ---------------------------Book - IT --> false CD - JAZZ --> true Cosmetics - Lipstick --> false CD - JAZZ IMPORTED --> true No of descriptions starts with CD -->2 ------------------- description longer than 15 characters -----------------------

174

How would you go about…?

Book - IT --> false CD - JAZZ --> false Cosmetics - Lipstick --> true CD - JAZZ IMPORTED --> true No of descriptions longer than 15 characters -->2

Scenario: So far so good, for illustration purpose if you need to adapt the strategy class to the CountDecorator class so that you do not have to explicitly cast your strategy classes to CountDecorator as shown in bold arrow in the class Shopping. We can overcome this by slightly rearranging the classes. The class CountDecorator has two additional methods count() and reset(). If you only just add these methods to the interface CheckStrategy then the classes LongerThan15 and StartsWithCD should provide an implementation for these two methods. These two methods make no sense in these two classes. Solution: So, to overcome this you can introduce an adapter class named CheckStrategyAdapter, which just provides a bare minimum default implementation. public interface CheckStrategy { public boolean check(String word); public int count(); public void reset(); } /** * This is an adapter class which provides default implementations to be extended not to be used and facilitates its subclasses to be * adapted to each other. Throws an unchecked exception to indicate improper use. */ public class CheckStrategyAdapter implements CheckStrategy { public boolean check(String word) { throw new RuntimeException("Improper use of CheckStrategyAdapter class method check(String word)" ); } public int count() { throw new RuntimeException("Improper use of CheckStrategyAdapter class method count()" ); } public void reset() { throw new RuntimeException("Improper use of CheckStrategyAdapter class method reset()" ); } } public class LongerThan15 extends CheckStrategyAdapter { public static final int LENGTH = 15; public boolean check(String description) { if (description == null) return false; else return description.length() > LENGTH; } } public class StartsWithCD extends CheckStrategyAdapter { public static final String STARTS_WITH = "cd"; public boolean check(String description) { String s = description.toLowerCase(); if (description == null || description.length() == 0) return false; else return s.startsWith(STARTS_WITH); } } public class CountDecorator extends CheckStrategyAdapter { private CheckStrategy cs = null; private int count = 0; public CountDecorator(CheckStrategy cs) { this.cs = cs; }

How would you go about…?

public boolean check(String description) { boolean isFound = cs.check(description); if (isFound) this.count++; return isFound; }

1 1 CountDecorator

public int count() { return this.count; } public void reset() { this.count = 0; } }

+check() : boolean +count() : int +reset()

«interface» CheckStrategy +check() : boolean +count() : int +reset() LongerThan15 CheckStrategyAdapter +check() : boolean +count() : int +reset()

+check() : boolean StartsWuthCD +check() : boolean

Adapter provides default implementation, so that it can be extended to provide specific implementation.

Now, let’s see the revised calling class Shopping: //...package & import statements public class Shopping { //...... public static void process() throws ItemException { //--------------------------Strategy and decorator design pattern---------------------------------System.out.println("----count item description starting with 'cd'or longer than 15 characters ---"); Iterator it = builder.getIterator(); boolean bol = false; CheckStrategy strategy = null; System.out.println("-------------------- description satarting with cd ----------------------------"); it = builder.getIterator(); //for starting with CD strategy = new StartsWithCD(); strategy = new CountDecorator(strategy); while (it.hasNext()) { Item item = (Item) it.next(); bol = strategy.check(item.getDescription()); System.out.println(item.getDescription() + " --> " + bol); } System.out.println("No of descriptions starts with CD -->" + strategy.count()); System.out.println("------------------- description longer than 15 characters -----------------------"); it = builder.getIterator(); //for starting with CD strategy = new LongerThan15(); strategy = new CountDecorator(strategy); while (it.hasNext()) { Item item = (Item) it.next(); bol = strategy.check(item.getDescription()); System.out.println(item.getDescription() + " --> " + bol); } System.out.println("No of descriptions longer than 15 characters -->" + strategy.count()); } } The output is: ----count item description starting with 'cd'or longer than 15 characters ---------------------- description satarting with cd ---------------------------Book - IT --> false CD - JAZZ --> true Cosmetics - Lipstick --> false CD - JAZZ IMPORTED --> true No of descriptions starts with CD -->2 ------------------- description longer than 15 characters ----------------------Book - IT --> false CD - JAZZ --> false Cosmetics - Lipstick --> true CD - JAZZ IMPORTED --> true

175

176

How would you go about…?

No of descriptions longer than 15 characters -->2

Scenario: The XYZ Retail also requires a piece of code, which performs different operations depending on the type of item. If the item is an instance of CD then you call a method to print its catalog number. If the item is an instance of Cosmetics then you call a related but different method to print its colour code. If the item is an instance of Book then you call a separate method to print its ISBN number. One way of implementing this is using the Java constructs instanceof and explicit type casting as shown below: it = builder.getIterator(); while(it.hasNext(); ) { String name = null; Item item = (Item)iter.next(); if(item instanceof CD) { ((CD) item). markWithCatalogNumber(); } else if (item instanceof Cosmetics) { ((Cosmetics) item). markWithColourCode (); } else if (item instanceof Book) { ((Book) item). markWithISBNNumber(); } }

Solution: The manipulation of a collection of polymorphic objects with the constructs typecasts and instanceof as shown above can get messy and unmaintainable with large elseif constructs and these constructs in frequently accessed methods/ loops can adversely affect performance. You can apply the visitor design pattern to avoid using these typecast and “instanceof” constructs as shown below: Visitor pattern: The visitor pattern makes adding new operations easy and all the related operations are localized in a visitor. The visitor pattern allows you to manipulate a collection of polymorphic objects without the messy and unmaintainable typecasts and instanceof operations. Visitor pattern allows you to add new operations, which affect a class hierarchy without having to change any of the classes in the hierarchy. For example we can add a GoodsDebugVisitor class to have the visitor just print out some debug information about each item visited etc. In fact you can write any number of visitor classes for the Goods hierarchy e.g. GoodsLabellingVisitor, GoodsXXXXVisitor etc. public interface Item { //... public void accept(ItemVisitor visitor); } public interface ItemVisitor { public void visit (CD cd); public void visit (Cosmetics cosmetics); public void visit (Book book); } /** * visitor class which calls different methods depending * on type of item. */ public class GoodsLabellingVisitor implements ItemVisitor {

«interface» ItemVisitor +visit(CD cd)() +visit(Book book)() +visit(Cosmetics cosmetics)()

GoodsLabellingVisitor

+visit(CD cd)() +visit(Book book)() +visit(Cosmetics cosmetics()

public void visit(CD cd) { markWithCatalogNumber(cd); } public void visit(Cosmetics cosmetics) { markWithColorNumber(cosmetics); }

«interface» Item +accept(ItemVisitor visitor)()

CD

Book

Cosmetics

+accept(ItemVisitor visitor)()

+accept(ItemVisitor visitor)()

+accept(ItemVisitor visitor)()

public void visit(Book book) { markWithISBNNumber(book); } private void markWithCatalogNumber(CD cd) { System.out.println("Catalog number for : " + cd.getDescription()); } private void markWithColorNumber(Cosmetics cosmetics) {

Goods

How would you go about…?

177

System.out.println("Color number for : " + cosmetics.getDescription()); } public void markWithISBNNumber(Book book) { System.out.println("ISBN number for : " + book.getDescription()); } } public class CD extends Goods { //... public void accept(ItemVisitor visitor) { visitor.visit(this); } } public class Book extends Goods { //... public void accept(ItemVisitor visitor) { visitor.visit(this); } } public class Cosmetics extends Goods { //... public void accept(ItemVisitor visitor) { visitor.visit(this); } }

Now, let’s see the calling code or class Shopping: //... package and import statements public class Shopping { public static void process() throws ItemException { //visitor pattern example, no messy instanceof and typecast constructs it = builder.getIterator(); ItemVisitor visitor = new GoodsLabellingVisitor (); while (it.hasNext()) { Item item = (Item) it.next(); item.accept(visitor); } } } The output is: ---- markXXXX(): avoid huge if else statements, instanceof & type casts -------ISBN number for : Book - IT Catalog number for : CD - JAZZ Color number for : Cosmetics - Lipstick Catalog number for : CD - JAZZ IMPORTED

Scenario: The XYZ Retail would like to have a functionality to iterate through every second or third item in the basket to randomly collect some statistics on price. Solution: This can be implemented by applying the iterator design pattern. Iterator pattern: Provides a way to access the elements of an aggregate object without exposing its underlying implementation. //… package and import statements public interface ItemBuilder { //.. public com.item.Iterator getItemIterator(); } package com.item;

178

How would you go about…?

public interface Iterator { public Item nextItem(); public Item previousItem(); public Item currentItem(); public Item firstItem(); public Item lastItem(); public boolean isDone(); public void setStep(int step); } //… package and import statements public class ShoppingBasketBuilder implements ItemBuilder { private List listItems = null;

« in te rfa c e » Ite m B u ild e r + g e tIte m Ite ra to r()

public Iterator getIterator() { return listItems.iterator(); } public com.item.Iterator getItemIterator() { return new ItemsIterator(); } /** * inner class which iterates over basket of items */ class ItemsIterator implements com.item.Iterator { private int current = 0; private int step = 1; public Item nextItem() { Item item = null; current += step; if (!isDone()) { item = (Item) listItems.get(current); } return item; } public Item previousItem() { Item item = null; current -= step; if (!isDone()) { item = (Item) listItems.get(current); } return item; } public Item firstItem() { current = 0; return (Item) listItems.get(current); } public Item lastItem() { current = listItems.size() - 1; return (Item) listItems.get(current); } public boolean isDone() { return current >= listItems.size() ? true : false; } public Item currentItem() { if (!isDone()) { return (Item) listItems.get(current); } else { return null; } }

S h o p p in g B a s k e tB u ild e r + g e tIte m Ite ra to r()

« in te rfa c e » Ite ra to r + c u rre n tIte m ()() + n e x tIte m () + p re v io u s Ite m () + firs tIte m () + la s tIte m ()

Ite m s Ite ra to r + c u rre n tIte m () + n e x tIte m () + p re v io u s Ite m () + firs tIte m () + la s tIte m ()

How would you go about…?

179

public void setStep(int step) { this.step = step; } } }

Now, let’s see the calling code Shopping: //… package & import statements public class Shopping { //.. public static void process() throws ItemException { //Iterator pattern example, inner implementations of ShopingBasketBuilder is protected. com.item.Iterator itemIterator = builder.getItemIterator(); //say we want to traverse through every second item in the basket itemIterator.setStep(2); Item item = null; for (item = itemIterator.firstItem(); !itemIterator.isDone(); item = itemIterator.nextItem()) { System.out.println("nextItem: " + item.getDescription() + " ====> " + item.getExtendedTaxPrice()); } item = itemIterator.lastItem(); System.out.println("lastItem: " + item.getDescription() + " ====> " + item.getExtendedTaxPrice()); item = itemIterator.previousItem(); System.out.println("previousItem : " + item.getDescription() + "====>" + item.getExtendedTaxPrice()); } } The output is: --------------- steps through every 2nd item in the basket ----------------------nextItem: Book - IT ====> 12.0 nextItem: Cosmetics - Lipstick ====> 1.1 lastItem: CD - JAZZ IMPORTED ====> 17.25 previousItem : CD - JAZZ====>16.5

Scenario: The XYZ Retail buys the items in bulk from warehouses and sells them in their retail stores. All the items sold need to be prepared for retail prior to stacking in the shelves for trade. The preparation involves 3 steps for all types of items, i.e. adding the items to stock in the database, applying barcode to each item and finally marking retail price on the item. The preparation process is common involving 3 steps but each of these individual steps is specific to type of item i.e. Book, CD, and Cosmetics. Solution: The above functionality can be implemented applying the template method design pattern as shown below: Template method pattern: When you have a sequence of steps to be processed within a method and you want to defer some of the steps to its subclass then you can use a template method pattern. So the template method lets the subclass to redefine some of the steps. Example Good example of this is the process() method in the Struts RequestProcessor class, which executes a sequence of processXXXX(…) methods allowing the subclass to override some of the methods when required. Refer Q110 in Enterprise section. Goods //... public abstract class Goods implements Item { //... /** * The template method */ public void prepareItemForRetai() { addToStock(); applyBarcode(); markRetailPrice(); } public abstract void addToStock();

+prepareItemForRetail() +addToStock() +applyBarcode() +markRetailPrice()

addToStock() --> abstract applyBarcode --> abstract markRetailPrice --> abstract

CD

Book

Cosmetics

+addToStock() +applyBarcode() +markRetailPrice()

+addToStock() +applyBarcode() +markRetailPrice()

+addToStock() +applyBarcode() +markRetailPrice()

180

How would you go about…?

public abstract void applyBarcode(); public abstract void markRetailPrice(); } //.. public class Book extends Goods { //.. //following methods gets called by the template method public void addToStock() { //database call logic to store the book in stock table. System.out.println("Book added to stock : " + this.getDescription()); } public void applyBarcode() { //logic to print and apply the barcode to book. System.out.println("Bar code applied to book : " + this.getDescription()); } public void markRetailPrice() { //logic to read retail price from the book table and apply the retail price. System.out.println("Mark retail price for the book : " + this.getDescription()); } } //... public class CD extends Goods { //.. //following methods gets called by the template method public void addToStock() { //database call logic to store the cd in stock table. System.out.println("CD added to stock : " + this.getDescription()); } public void applyBarcode() { //logic to print and apply the barcode to cd. System.out.println("Bar code applied to cd : " + this.getDescription()); } public void markRetailPrice() { //logic to read retail price from the cd table and apply the retail price. System.out.println("Mark retail price for the cd : " + this.getDescription()); } } //... public class Cosmetics extends Goods { //... public void addToStock() { //database call logic to store the cosmetic in stock table. System.out.println("Cosmetic added to stock : " + this.getDescription()); } public void applyBarcode() { //logic to print and apply the barcode to cosmetic. System.out.println("Bar code applied to cosmetic : " + this.getDescription()); } public void markRetailPrice() { //logic to read retail price from the cosmetic table and apply the retail price. System.out.println("Mark retail price for the cosmetic : " + this.getDescription()); } }

Now, let’s see the calling code Shopping: //... public class Shopping { //...

How would you go about…?

181

public static void process() throws ItemException { //... Item item = null; for (item = itemIterator.firstItem(); !itemIterator.isDone(); item = itemIterator.nextItem()) { item.prepareItemForRetai(); System.out.println("-----------------------------------"); } } } The output is: ------------------- prepareItemForRetail() ------------------------------Book added to stock : Book - IT Bar code applied to book : Book - IT Mark retail price for the book : Book - IT

Scenario: The employees of XYZ Retail are at various positions. In a hierarchy, the general manager has subordinates, and also the sales manager has subordinates. The retail sales staffs have no subordinates and they report to their immediate manager. The company needs functionality to calculate salary at different levels of the hierarchy. Solution: You can apply the composite design pattern to represent the XYZ Retail company employee hierarchy. Composite design pattern: The composite design pattern composes objects into tree structures where individual objects like sales staff and composite objects like managers are handled uniformly. Refer Q52 in Java section or Q25 in Enterprise section. /** * Base employee class */ public abstract class Employee { private String name; private double salary;

E m ployee

public Employee(String name, double salary) { this.name = name; this.salary = salary; }

+addE m ployee() +rem oveE m ployee() +hasS ubordinates() +getS alaries()

public String getName() { return name; } public double getSalaries() { return salary; } public abstract boolean addEmployee(Employee emp); public abstract boolean removeEmployee(Employee emp); protected abstract boolean hasSubordinates();

*

Staff

M anag er

+addE m ployee() +rem oveE m ployee() +hasS ubordinates()

+addE m ployee() +rem oveEm ployee() +hasSubordinates() +getSalaries()

} Leaf // package & import statements /** * This is the Employee composite class having subordinates. */ public class Manager extends Employee { List subordinates = null; public Manager(String name, double salary) { super(name, salary); } public boolean addEmployee(Employee emp) { if (subordinates == null) { subordinates = new ArrayList(10); }

C om posite

1

182

How would you go about…?

return subordinates.add(emp); } public boolean removeEmployee(Employee emp) { if (subordinates == null) { subordinates = new ArrayList(10); } return subordinates.remove(emp); } /** * Recursive method call to calculate the sum of salary of a manager and his subordinates, which means sum of salary of a manager * on whom this method was invoked and any employees who themselves will have any subordinates and so on. */ public double getSalaries() { double sum = super.getSalaries(); //this one's salary if (this.hasSubordinates()) { for (int i = 0; i < subordinates.size(); i++) { sum += ((Employee) subordinates.get(i)).getSalaries(); } } return sum; } public boolean hasSubordinates() { boolean hasSubOrdinates = false; if (subordinates != null && subordinates.size() > 0) { hasSubOrdinates = true; } return hasSubOrdinates; } } /** * This is the leaf staff employee object. staff do not have any subordinates. */ public class Staff extends Employee { public Staff(String name, double salary) { super(name, salary); } public boolean addEmployee(Employee emp) { throw new RuntimeException("Improper use of Staff class"); } public boolean removeEmployee(Employee emp) { throw new RuntimeException("Improper use of Staff class"); } protected boolean hasSubordinates() { return false; } }

Now, let’s see the calling code Shopping: //... public class Shopping { //..... public static void process() throws ItemException { //.... System.out.println("--------------------- Employee hierachy & getSalaries() recursively -------------"); //Employee hierachy Employee generalManager = new Manager("John Smith", 100000.00); Employee salesManger = new Manager("Peter Rodgers", 80000.00); Employee logisticsManger = new Manager("Graham anthony", 90000.00);

How would you go about…?

183

Employee staffSales1 = new Staff("Lisa john", 40000.00); Employee staffSales2 = new Staff("Pamela watson", 50000.00); salesManger.addEmployee(staffSales1); salesManger.addEmployee(staffSales2); Employee logisticsTeamLead = new Manager("Cooma kumar", 70000.00); Employee staffLogistics1 = new Staff("Ben Sampson", 60000.00); Employee staffLogistics2 = new Staff("Vincent Chou", 20000.00); logisticsTeamLead.addEmployee(staffLogistics1); logisticsTeamLead.addEmployee(staffLogistics2); logisticsManger.addEmployee(logisticsTeamLead); generalManager.addEmployee(salesManger); generalManager.addEmployee(logisticsManger); System.out.println(staffSales1.getName() + "-->" + staffSales1.getSalaries()); System.out.println(staffSales2.getName() + "-->" + staffSales2.getSalaries()); System.out.println("Logistics dept " + " --> " + logisticsManger.getSalaries()); System.out.println("General Manager " + " --> " + generalManager.getSalaries()); } } The output is: --------------------- Employee hierachy & getSalaries() recursively ------------Lisa john-->40000.0 Pamela watson-->50000.0 Logistics dept --> 240000.0 General Manager --> 510000.0

Scenario: The purchasing staffs (aka logistics staff) of the XYZ Retail Company need to interact with other subsystems in order to place purchase orders. They need to communicate with their stock control department to determine the stock levels, also need to communicate with their wholesale supplier to determine availability of stock and finally with their bank to determine availability of sufficient funds to make a purchase. Solution: You can apply the façade design pattern to implement the above scenario. Façade pattern: The façade pattern provides an interface to large subsystems of classes. A common design goal is to minmize the communication and dependencies between subsystems. One way to achieve this goal is to introduce a façade object that provides a single, simplified interface. W ith o u t fa c a d e

public class StockControl { public boolean isBelowReorderpoint(Item item) { //logic to evaluate stock level for item return true; } } public class Bank { public boolean hasSufficientFunds() { //logic to evaluate if we have sufficient savings goes here return true; } }

B ank

S to c k C o n tro l

W h o le S a le r

W ith fa c a d e

public class WholeSaler { public boolean hasSufficientStock(Item item) { //logic to evaluate if the wholesaler has enough stock goes here return true; } } P u r c h a s e E v a lu a tio n F a c a d e

/** * This is the facade class */ public class PurchaseEvaluation { private StockControl stockControl = new StockControl();

B an k

S to c k C o n tro l

W h o le s a le r

How would you go about…?

184

private WholeSaler wholeSaler = new WholeSaler(); private Bank bank = new Bank(); public boolean shouldWePlaceOrder(Item item) { if (!stockControl.isBelowReorderpoint(item)) { return false; } if (!wholeSaler.hasSufficientStock(item)) { return false; } if (!bank.hasSufficientFunds()) { return false; } return true; } }

Now, let’s see the calling code or class Shopping: //.... public class Shopping { //....... public static void process() throws ItemException { //.... //----------------------facade design pattern -------------------------------System.out.println("--------------------shouldWePlaceOrder----------------------------") ; PurchaseEvaluation purchaseEval = new PurchaseEvaluation(); boolean shouldWePlaceOrder = purchaseEval.shouldWePlaceOrder(item); System.out.println("shouldWePlaceOrder=" + shouldWePlaceOrder); } } The output is: --------------------shouldWePlaceOrder()---------------------------shouldWePlaceOrder=true

Scenario: The purchasing deprtment also requires functionality where, when the stock control system is updated, all the registered departmental systems like logistics and sales should be notified of the change. Solution: This can be achieved by applying the observer design pattern as shown below: Observer pattern: defines a one-to-many dependency between objects so that when one object changes state, all its dependents are notified and updated automatically. (aka publish-subscribe pattern) O b s e rv e rs 1 s t Q t r 1 0 0

2 nd Q t r

5 0 0 1 s t Q t r

E a s t

3 r d Q t r

W e s t

4 t h Q t r

N o r t h

3 r d Q t r

ch Not an ify ge

re g

is t

er

g re

is t

er

y tif ge No an ch

1 s t Q tr = 1 0 % , 2 n d Q tr= 2 0 % , 3 Q tr = 6 0 % , 4 th Q tr = 1 0 %

S u b je c t /** * This is an observer (aka subscriber) interface. This gets notified through its update method. */ public interface Department { public void update(Item item, int qty); }

How would you go about…?

185

public class LogisticsDepartment implements Department { public void update(Item item, int qty) { //logic to update department's stock goes here System.out.println("Logistics has updated its stock for " + item.getDescription() + " with qty=" + qty); } } public class SalesDepartment implements Department { public void update(Item item, int qty) { //logic to update department's stock goes here System.out.println("Sales has updated its stock for " + item.getDescription() + " with qty=" + qty); } } /** * Subject (publisher) class: when stock is updated, notifies all the * subscribers. */ public interface StockControl { public void notify(Item item, int qty); public void updateStock(Item item, int qty) ; public boolean addSubscribers(Department dept); public boolean removeSubscribers(Department dept); } //… package & import statements ** * publisher (observable) class: when stock is updated * notifies all the subscribers. */ public class XYZStockControl implements StockControl{

«interface» StockControl +addSubscribers() +removeSubscribers() +notify()

-observers

«interface» Department +update()

Observer (aka Subscriber)

List listSubscribers = new ArrayList(10); //... public boolean addSubscribers(Department dept) { return listSubscribers.add(dept); } public boolean removeSubscribers(Department dept) { return listSubscribers.remove(dept); }

XYZStockControl LogisticsDepartment SalesDepartment +addSubscribers() +removeSubscribers() +notify()

Subject (aka publisher)

/** * writes updated stock qty into databases */ public void updateStock(Item item, int qty) { //logic to update an item's stock goes here notify(item, qty); //notify subscribers that with the updated stock info. } public void notify(Item item, int qty) { int noOfsubscribers = listSubscribers.size(); for (int i = 0; i < noOfsubscribers; i++) { Department dept = (Department) listSubscribers.get(i); dept.update(item, qty); } } }

Now, let’s see the calling code or class Shopping: // package & import statements public class Shopping { //............... public static void process() throws ItemException { //......... //----------------------observer design pattern-------------------------------------------

-subject+update()

+update()

How would you go about…?

186

System.out.println("--------------------notify stock update----------------------------"); Department deptLogistics = new LogisticsDepartment(); //observer/subscriber Department salesLogistics = new SalesDepartment(); //observer/subscriber StockControl stockControl = new XYZStockControl();//observable/publisher //let's register subscribers with the publisher stockControl.addSubscribers(deptLogistics); stockControl.addSubscribers(salesLogistics); //let's update the stock value of the publisher for (item = itemIterator.firstItem(); !itemIterator.isDone(); item = itemIterator.nextItem()) { if (item instanceof CD) { stockControl.updateStock(item, 25); } else if (item instanceof Book){ stockControl.updateStock(item, 40); } else { stockControl.updateStock(item, 50); } } } } The output is: --------------------notify stock update---------------------------Logistics has updated its stock for Book - IT with qty=40 Sales has updated its stock for Book - IT with qty=40 Logistics has updated its stock for CD - JAZZ with qty=25 Sales has updated its stock for CD - JAZZ with qty=25 Logistics has updated its stock for Cosmetics - Lipstick with qty=50 Sales has updated its stock for Cosmetics - Lipstick with qty=50 Logistics has updated its stock for CD - JAZZ IMPORTED with qty=25 Sales has updated its stock for CD - JAZZ IMPORTED with qty=25

Scenario: The stock control staffs require a simplified calculator, which enable them to add and subtract stock counted and also enable them to undo and redo their operations. This calculator will assist them with faster processing of stock counting operations. Solution: This can be achieved by applying the command design pattern as shown below: Command pattern: The Command pattern is an object behavioural pattern that allows you to achieve complete decoupling between the sender and the receiver. (A sender is an object that invokes an operation, and a receiver is an object that receives the request to execute a certain operation. With decoupling, the sender has no knowledge of the Receiver's interface.) The term request here refers to the command that is to be executed. The Command pattern also allows you to vary when and how a request is fulfilled. At times it is necessary to issue requests to objects without knowing anything about the operation being requested or the receiver of the request. In procedural languages, this type of communication is accomplished via a call-back: a function that is registered somewhere to be called at a later point. Commands are the object-oriented equivalent of call-backs and encapsulate the call-back function. C o m m a n d p a tte rn

3 a c a

. in v o k e t h e in v o k e r n d p a s s th e om m and as an rg u m e n t

T e st ( c lie n t a p p lic a t io n )

D r a w In v o k e r (In v o k e r ) + d ra w ()

2 c p a

. o a s

in v o k e th e m m and and s s th e r e c ie v e r a n a rg u m e n t

1 . in v o k e th e r e c e iv e r

« in te r fa c e » C o m m a n d 1

*

+ e x e c u te ()

C ir c le C o m m a n d

S q u a re C o m m a n d

+ e x e c u te ( )

+ e x e c u te ()

C ir c le + d ra w ()

S q u a re + d ra w ()

How would you go about…?

187

// package & import statements /** * Invoker */ public class Staff extends Employee { private Calculator calc = new Calculator(); private List listCommands = new ArrayList(15); private int current = 0; public Staff(String name) { super(name); } //... /** * make use of command. */ public void compute(char operator, int operand) { Command command = new CalculatorCommand(calc, operator, operand);//initialise the calculator command.execute(); //add commands to the list so that undo operation can be performed listCommands.add(command); current++; } /** * perform redo operations */ public void redo(int noOfLevels) { int noOfCommands = listCommands.size(); for (int i = 0; i < noOfLevels; i++) { if (current < noOfCommands) { ((Command) listCommands.get(current++)).execute(); } } } /** * perform undo operations */ public void undo(int noOfLevels) { for (int i = 0; i < noOfLevels; i++) { if (current > 0) { ((Command) listCommands.get(--current)).unexecute(); } } } } ** * actual receiver of the command who performs calculation */ public class Calculator { private int total = 0; /** * calculates. */ public void calculate(char operator, int operand) { switch (operator) { case '+': Invoker total += operand; break; case '-': total -= operand; break; }

Employee

Staff +compute() +redo() +undo()

Calculator System.out.println("Total = " + total); }

Receiver +calculate()

1

*

«interface» Command +execute() +unexecute()

CalculatorCommand +execute() +unexecute()

Command, which decouples the invoker from the receiver.

188

How would you go about…?

} /** * command interface */ public interface Command { public void execute(); public void unexecute(); } /** * calculator command, which decouples the receiver Calculator from the invoker staff */ public class CalculatorCommand implements Command { private Calculator calc = null; private char operator; private int operand; public CalculatorCommand(Calculator calc, char operator, int operand) { this.calc = calc; this.operator = operator; this.operand = operand; } public void execute() { calc.calculate(operator, operand); } public void unexecute() { calc.calculate(undoOperand(operator), operand); } private char undoOperand(char operator) { char undoOperator = ' '; switch (operator) { case '+': undoOperator = '-'; break; case '-': undoOperator = '+'; break; } return undoOperator; } }

Now, let’s see the calling code or class Shopping: //.............. public class Shopping { //........... public static void process() throws ItemException { //------------------------------------command design pattern---------------------------------------System.out.println("---------------Calculator with redo & undo operations---------------------------"); Staff stockControlStaff = new Staff("Vincent Chou"); stockControlStaff.compute('+',10);//10 stockControlStaff.compute('-',5);//5 stockControlStaff.compute('+',10);//15 stockControlStaff.compute('-',2);//13 //lets try our undo operations System.out.println("---------------undo operation : 1 level---------------------------"); stockControlStaff.undo(1); System.out.println("---------------undo operation :2 levels---------------------------"); stockControlStaff.undo(2); //lets try our redo operations System.out.println("---------------redo operation : 2 levels---------------------------"); stockControlStaff.redo(2);

How would you go about…?

189

System.out.println("---------------redo operation : 1 level---------------------------"); stockControlStaff.redo(1); } } The output is: --------------Calculator with redo & undo operations--------------------------Total = 10 Total = 5 Total = 15 Total = 13 ---------------undo operation : 1 level--------------------------Total = 15 ---------------undo operation :2 levels--------------------------Total = 5 Total = 10 ---------------redo operation : 2 levels--------------------------Total = 5 Total = 15 ---------------redo operation : 1 level--------------------------Total = 13

Scenario: The XYZ Retail has a 3rd party software component called XYZPriceList, which implements an interface rd PriceList. This 3 party software component is not thread-safe. So far it performed a decent job since only the sales manager had access to this software component. The XYZ Retail now wants to provide read and write access to all the managers. The source code is not available and only the API is available, so modifying the existing component is not viable. This will cause a dirty read problem if two managers try to concurrently access this component. For example, if the sales manager tries to access an item’s price while the logistics manger is modifying the price (say modification takes 1 second), then the sales manager will be reading the wrong value. Let’s look at this with a code sample: public interface PriceList { public double getPrice(int itemId) ; public void setPrice(int itemId,double newPrice) ; } //… public class XYZPriceList implements PriceList{ private static final Map mapPrices = new HashMap(30,.075f); public static PriceList singleInstance = new XYZPriceList();//only one instance /** * static initializer block */ static { //only one item is added to keep it simple mapPrices.put(new Integer(1), new Double(12.00));//Book - IT //... add more items to price list } public static PriceList getInstance() { return singleInstance; }

«interface» PriceList +getPrice() +setPrice()

XYZPriceList +getPrice() +setPrice()

public double getPrice(int itemId) { double price = ((Double)mapPrices.get(new Integer(itemId))).doubleValue(); System.out.println("The price of the itemId " + itemId + " = "+ price); return price; } public void setPrice(int itemId,double newPrice) { System.out.println("wait while mutating price from 12.0 to 15.00 ..........."); try { mapPrices.put(new Integer(itemId),new Double(-1));//transient value while updating with a proper value Thread.sleep(1000);//assume update/set operation takes 1 second mapPrices.put(new Integer(itemId),new Double(newPrice)); } catch (InterruptedException ie) {} } } The multi-threaded access class:

190

How would you go about…?

public class PriceListUser implements Runnable { int itemId; double price; static int count = 0; public PriceListUser(int itemId) { this.itemId = itemId; } /** * runnable code where multi-threads are executed */ public void run() { String name = Thread.currentThread().getName(); if (name.equals("accessor")) { rd price = XYZPriceList.getInstance().getPrice(itemId); //using 3 party commponent } else if (name.equals("mutator")) { rd XYZPriceList.getInstance().setPrice(itemId, 15.00); //using 3 party commponent } } }

Now, let’s see the calling code or class Shopping: //.... public class Shopping { //.... public static void process() throws ItemException { //.......... //------------------------------------proxy design pattern-----------------------------System.out.println("---------------Accessing the price list---------------------------"); PriceListUser user1 = new PriceListUser(1);//accessing same itemId=1 PriceListUser user2 = new PriceListUser(1);//accessing same itemId=1 Thread t1 = new Thread(user1); Thread t2 = new Thread(user2); Thread t3 = new Thread(user1); t1.setName("accessor");//user 1 reads the price t2.setName("mutator");//user 2 modifies the price t3.setName("accessor");//user 1 reads the price t1.start();//accessor user-1 reads before mutator user-2 modifies the price as 12.00 t2.start();//mutator user-2 sets the price to 15.00 t3.start();//while the user-2 is setting the price to 15.00 user-1 reads again and gets the price as 12.00 //user-2 gets the wrong price i.e gets 12.0 again instead of 15.00 } } The output is: ---------------Accessing the price list--------------------------The price of the itemId 1 = 12.0 wait while mutating price from 12.0 to 15.00 ........... The price of the itemId 1 = -1.0 OR ---------------Accessing the price list--------------------------wait while mutating price from 12.0 to 15.00 ........... The price of the itemId 1 = -1.0 The price of the itemId 1 = -1.0

Problem: You get one of the two outputs shown above depending on how the threads initialized by the operating system. The first value of 12.0 is okay and the second value of 12.0 again is a dirty read because the value should have been modified to 15.0 by the user-2. So the user-1 reading the value for the second time should get the value of 15.0 after it has been modified.

How would you go about…?

191

Solution: This threading issue and inability to modify the existing component can be solved by applying the proxy design pattern. You will be writing a proxy class, which will apply the locking for the entries in the XYZPriceList. This proxy class internally will be making use of the XYZPriceList in a synchronized fashion as shown below: Proxy pattern: Provides a surrogate or placeholder for another object to control access to it. Provide a surrogate or placeholder for another object to control access to it. Proxy object acts as an intermediary between the client and the target object. The proxy object has the same interface as the target object. The proxy object holds reference to the target object. There are different types of proxies: ƒ

Remote Proxy: provides a reference to an object, which resides in a separate address space. e.g. EJB, RMI, CORBA etc (RMI stubs acts as a proxy for the skeleton objects.)

ƒ

Virtual Proxy: Allows the creation of memory intensive objects on demand. The target object will not be created until it is really needed.

ƒ

Access Proxy: Provides different clients with different access rights to the target object.

Example In Hibernate framework (Refer Q15 - Q16 in Emerging Technologies/Frameworks section) lazy loading of persistent objects are facilitated by virtual proxy pattern. Say you have a Department object, which has a collection of Employee objects. Let’s say that Employee objects are lazy loaded. If you make a call department.getEmployees() then Hibernate will load only the employeeIDs and the version numbers of the Employee objects, thus saving loading of individual objects until later. So what you really have is a collection of proxies not the real objects. The reason being, if you have hundreds of employees for a particular department then chances are good that you will only deal with only a few of them. So, why unnecessarily instantiate all the Employee objects? This can be a big performance issue in some situations. So when you make a call on a particular employee i.e. employee.getName() then the proxy loads up the real object from the database. P ro x y p a tte rn Test (c lie n t a p p lic a tio n )

« in te rfa c e » S u b je c t + re q u e s t()

R e a lS u b je c t + re q u e s t()

a C lie n t s u b je c t

a P ro x y re a lS u b je c t

P ro x y + re q u e s t()

a R e a lS u b je c t

/** * synchronized proxy class for XYZPriceList */ public class XYZPriceListProxy implements PriceList { //assume that we only have two items in the pricelist Integer[] locks = { new Integer(1), new Integer(2) };//locks for each item in the price list

« in te rfa c e » P ric e L is t + g e tP ric e () + se tP ric e ()

public static PriceList singleInstance = new XYZPriceListProxy();//single instance of XYZPriceListProxy PriceList realPriceList = XYZPriceList.getInstance(); // real object public static PriceList getInstance() { return singleInstance; } public double getPrice(int itemId) { synchronized (locks[itemId]) { return realPriceList.getPrice(itemId); } } public void setPrice(int itemId, double newPrice) {

X Y Z P ric e L is tP ro x y + g e tP ric e () + s e tP ric e ()

P ro xy (a k a su rro g a te )

-re a lS u b je c t

X Y Z P ric e L is t + g e tP ric e () + se tP ric e ()

re a l su b je c t

How would you go about…?

192 synchronized (locks[itemId]) { realPriceList.setPrice(itemId, newPrice); } } }

You should make a slight modification to the PriceListUser class as shown below in bold. public class PriceListUser implements Runnable { int itemId; double price; static int count = 0; public PriceListUser(int itemId) { this.itemId = itemId; } /** * runnable code where multi-threads are executed */ public void run() { String name = Thread.currentThread().getName(); if (name.equals("accessor")) { price = XYZPriceListProxy.getInstance().getPrice(itemId); } else if (name.equals("mutator")) { XYZPriceListProxy.getInstance().setPrice(itemId, 15.00); } } } Running the same calling code Shopping will render the following correct results by preventing dirty reads: ---------------Accessing the price list--------------------------The price of the itemId 1 = 12.0 wait while mutating price from 12.0 to 15.00 ........... The price of the itemId 1 = 15.0 OR ---------------Accessing the price list--------------------------wait while mutating price from 12.0 to 15.00 ........... The price of the itemId 1 = 15.0 The price of the itemId 1 = 15.0

What is a dynamic proxy? Dynamic proxies were introduced in J2SE 1.3, and provide an alternate dynamic mechanism for implementing many common design patterns like Façade, Bridge, Decorator, Proxy (remote proxy and virtual proxy), and Adapter. While all of these patterns can be written using ordinary classes instead of dynamic proxies, in many situations dynamic proxies are more compact and can eliminate the need for a lot of handwritten classes. Dynamic proxies are reflection-based and allow you to intercept method calls so that you can interpose additional behaviour between a class caller and its callee. Dynamic proxies are not always appropriate because this code simplification comes at a performance cost due to reflection overhead. Dynamic proxies illustrate the basics of Aspect Oriented Programming (AOP) which complements your Object Oriented Programming. Refer Q03, Q04 and Q05 in Emerging Technologies/Frameworks section. Where can you use dynamic proxies? Dynamic proxies can be used to add crosscutting concerns like logging, performance metrics, memory logging, retry semantics, test stubs, caching etc. Let’s look at an example: InvocationHandler interface is the heart of a proxy mechanism. import java.lang.reflect.InvocationHandler; import java.lang.reflect.InvocationTargetException; import java.lang.reflect.Method; /** * Handles logging and invocation of target method */ public class LoggingHandler implements InvocationHandler { protected Object actual; public LoggingHandler(Object actual) {

How would you go about…? this.actual = actual; } public Object invoke(Object proxy, Method method, Object[] args) throws Throwable { try { System.out.println(">>>>>>start executing method: " + method.getName()); Object result = method.invoke(actual, args); return result; } catch (InvocationTargetException ite) { throw new RuntimeException(ite.getMessage()); } finally { System.out.println("